137MHz "V" Antenna advice needed ...


David Slipper
 

Hi, I am in the process of making a "horizontal 'V' dipole" antenna for receiving NOAA and have a question about adding a reflector.

It will have 8 elements in the reflector and my questions are :-

1. Do the 8 elements need to be connected to each other ???

2. Does the reflector need to be grounded ??? or perhaps connected to the coax braid in some way ???

I'm a newbie in all this so any advice is welcome.

Regards,
Dave


Kriss Kliegle KA1GJU
 

Slightly confused... any images of said 8 element horizontal V dipole?
With that said...
A beam (Yagi) with 8 elements will be highly directional, not something you want when chasing a satellite that is moving across the sky... unless you plan on using an azimuth/elevation rotator (more complexity).
I know one satellite operator in the club uses the 'egg beater' type antennas, they're omnidirectional  and no there are no moving parts to fail.

Typically in a beam antenna, the elements are +/- 1/2 wave long (two +/- 1/4 wave elements connected together to form one +/- 1/2 wavelength long element). Since the center of each parasitic element (essentially a dipole) is at 0 volts, the element is usually clamped to a metal boom without being insulated.
Lots of info on the web to get dimensions of each element and the spacing (google "VHF beam antenna dimensions" or similar)

i.e: https://www.rfwireless-world.com/calculators/3-element-Yagi-Antenna-Calculator.html
Yagi Antenna calculator Formula
Yagi beam antennas are great for narrowing the 'field of view' of RF, but once not aimed within the beam width, the RX'ed signal can drop off dramatically.
Antenna specs of commercially available antennas will state what the half-power beam width is. For example a -3dB drop or half power drop from the main lobe:

Antenna Beamwidth

The more elements added, the more forward gain (better front to back ratio)... at a cost of a more narrow beam width. Of course there are lots of variables that can be tweaked, height above ground, element spacing, element lengths, etc to change the specs of a beam.

Confused yet?

73 Kriss KA1GJU/AM


David Slipper
 

It's NOT a Yagi - this picture (see HERE)  is similar to what I am building but for 137MHz (the one in the pic is for a higher frequency).

The two dipole elements are horizontal and 120 degrees apart.

Do the reflector elements need to be connected together??

Do they need grounding ?? or connecting to the feed in some way ??

Dave



On 21/12/2021 23:30, Kriss Kliegle KA1GJU wrote:

Slightly confused... any images of said 8 element horizontal V dipole?
With that said...
A beam (Yagi) with 8 elements will be highly directional, not something you want when chasing a satellite that is moving across the sky... unless you plan on using an azimuth/elevation rotator (more complexity).
I know one satellite operator in the club uses the 'egg beater' type antennas, they're omnidirectional  and no there are no moving parts to fail.

Typically in a beam antenna, the elements are +/- 1/2 wave long (two +/- 1/4 wave elements connected together to form one +/- 1/2 wavelength long element). Since the center of each parasitic element (essentially a dipole) is at 0 volts, the element is usually clamped to a metal boom without being insulated.
Lots of info on the web to get dimensions of each element and the spacing (google "VHF beam antenna dimensions" or similar)

i.e: https://www.rfwireless-world.com/calculators/3-element-Yagi-Antenna-Calculator.html
Yagi Antenna calculator Formula
Yagi beam antennas are great for narrowing the 'field of view' of RF, but once not aimed within the beam width, the RX'ed signal can drop off dramatically.
Antenna specs of commercially available antennas will state what the half-power beam width is. For example a -3dB drop or half power drop from the main lobe:

Antenna Beamwidth

The more elements added, the more forward gain (better front to back ratio)... at a cost of a more narrow beam width. Of course there are lots of variables that can be tweaked, height above ground, element spacing, element lengths, etc to change the specs of a beam.

Confused yet?

73 Kriss KA1GJU/AM


jdow
 

I am making educated guesses here.

Yes, connect them together to the shield of the feed line. I am not sure what you are trying to achieve here, though.

{^_^}

On 20211221 19:04:08, David Slipper wrote:

It's NOT a Yagi - this picture (see HERE)  is similar to what I am building but for 137MHz (the one in the pic is for a higher frequency).

The two dipole elements are horizontal and 120 degrees apart.

Do the reflector elements need to be connected together??

Do they need grounding ?? or connecting to the feed in some way ??

Dave



On 21/12/2021 23:30, Kriss Kliegle KA1GJU wrote:

Slightly confused... any images of said 8 element horizontal V dipole?
With that said...
A beam (Yagi) with 8 elements will be highly directional, not something you want when chasing a satellite that is moving across the sky... unless you plan on using an azimuth/elevation rotator (more complexity).
I know one satellite operator in the club uses the 'egg beater' type antennas, they're omnidirectional  and no there are no moving parts to fail.

Typically in a beam antenna, the elements are +/- 1/2 wave long (two +/- 1/4 wave elements connected together to form one +/- 1/2 wavelength long element). Since the center of each parasitic element (essentially a dipole) is at 0 volts, the element is usually clamped to a metal boom without being insulated.
Lots of info on the web to get dimensions of each element and the spacing (google "VHF beam antenna dimensions" or similar)

i.e: https://www.rfwireless-world.com/calculators/3-element-Yagi-Antenna-Calculator.html
Yagi Antenna calculator Formula
Yagi beam antennas are great for narrowing the 'field of view' of RF, but once not aimed within the beam width, the RX'ed signal can drop off dramatically.
Antenna specs of commercially available antennas will state what the half-power beam width is. For example a -3dB drop or half power drop from the main lobe:

Antenna Beamwidth

The more elements added, the more forward gain (better front to back ratio)... at a cost of a more narrow beam width. Of course there are lots of variables that can be tweaked, height above ground, element spacing, element lengths, etc to change the specs of a beam.

Confused yet?

73 Kriss KA1GJU/AM


Michael Durkin
 

DiskCone is what I see ... 


On Tue, Dec 21, 2021, 7:04 PM David Slipper <softfoot@...> wrote:

It's NOT a Yagi - this picture (see HERE)  is similar to what I am building but for 137MHz (the one in the pic is for a higher frequency).

The two dipole elements are horizontal and 120 degrees apart.

Do the reflector elements need to be connected together??

Do they need grounding ?? or connecting to the feed in some way ??

Dave



On 21/12/2021 23:30, Kriss Kliegle KA1GJU wrote:

Slightly confused... any images of said 8 element horizontal V dipole?
With that said...
A beam (Yagi) with 8 elements will be highly directional, not something you want when chasing a satellite that is moving across the sky... unless you plan on using an azimuth/elevation rotator (more complexity).
I know one satellite operator in the club uses the 'egg beater' type antennas, they're omnidirectional  and no there are no moving parts to fail.

Typically in a beam antenna, the elements are +/- 1/2 wave long (two +/- 1/4 wave elements connected together to form one +/- 1/2 wavelength long element). Since the center of each parasitic element (essentially a dipole) is at 0 volts, the element is usually clamped to a metal boom without being insulated.
Lots of info on the web to get dimensions of each element and the spacing (google "VHF beam antenna dimensions" or similar)

i.e: https://www.rfwireless-world.com/calculators/3-element-Yagi-Antenna-Calculator.html
Yagi Antenna calculator Formula
Yagi beam antennas are great for narrowing the 'field of view' of RF, but once not aimed within the beam width, the RX'ed signal can drop off dramatically.
Antenna specs of commercially available antennas will state what the half-power beam width is. For example a -3dB drop or half power drop from the main lobe:

Antenna Beamwidth

The more elements added, the more forward gain (better front to back ratio)... at a cost of a more narrow beam width. Of course there are lots of variables that can be tweaked, height above ground, element spacing, element lengths, etc to change the specs of a beam.

Confused yet?

73 Kriss KA1GJU/AM


Kriss Kliegle KA1GJU
 

Connected... Yes
Grounded... Yes (coax shield)

Satellites reception

But I'm currently in a Hilton, not a Holiday Inn... so I am not a rocket scientist yet, but maybe on the next trip?


73 Kriss KA1GJU/AM


David J Taylor
 

On 22/12/2021 03:04, David Slipper wrote:
It's NOT a Yagi - this picture (see HERE
<https://www.amsat.se/wordpress/wp-content/uploads/2020/06/V-dipole_ritning.jpg>)
is similar to what I am building but for 137MHz (the one in the pic is for a
higher frequency).

The two dipole elements are horizontal and 120 degrees apart.

Do the reflector elements need to be connected together??

Do they need grounding ?? or connecting to the feed in some way ??

Dave
Dave,

If you are going to build something the QFH antenna would be a better choice
for APT/LRPT 137 MHz reception, for example:

https://www.satsignal.eu/wxsat/equipment.htm

but search for more recent links.

David
--
SatSignal Software - Quality software for you
Web: https://www.satsignal.eu
Email: david-taylor@blueyonder.co.uk
Twitter: @gm8arv


Larry Horlick
 

That only works in a Holiday Inn Express. A regular Holiday Inn won't do it...

Larry

On Wed, Dec 22, 2021 at 12:14 AM Kriss Kliegle KA1GJU <kliegle@...> wrote:
Connected... Yes
Grounded... Yes (coax shield)

Satellites reception

But I'm currently in a Hilton, not a Holiday Inn... so I am not a rocket scientist yet, but maybe on the next trip?


73 Kriss KA1GJU/AM


D R
 

A lot of Daves on here giving advice!

The reflectors on the illustrated V antenna are simply forming a ground plane, and they should NOT be connected to the feedline.  Ideally, they should be connected to each other where they cross, or at least make each pair of opposites from a single length of wire/rod/tube.  Some experimentation with the distance between the V and the ground plane may be necessary, as this will affect the sensitivity at low elevations (with a standard turnstile antenna the best spacing is around 3/8 wavelength, but it may be different with a V).

As David Taylor says, a QFH antenna is the preferred modern antenna for WxSat reception, but if you are just starting out the V will be a lot simpler to make, and you can trade up later.  Have fun!

Regards,
Dave


David Slipper
 

That makes sense ... many thanks.

I have a box full of 750mm telescopic aerials so this was a very cheap starting point ;-)

Dave


On 22/12/2021 13:50, D R via groups.io wrote:
A lot of Daves on here giving advice!

The reflectors on the illustrated V antenna are simply forming a ground plane, and they should NOT be connected to the feedline.  Ideally, they should be connected to each other where they cross, or at least make each pair of opposites from a single length of wire/rod/tube.  Some experimentation with the distance between the V and the ground plane may be necessary, as this will affect the sensitivity at low elevations (with a standard turnstile antenna the best spacing is around 3/8 wavelength, but it may be different with a V).

As David Taylor says, a QFH antenna is the preferred modern antenna for WxSat reception, but if you are just starting out the V will be a lot simpler to make, and you can trade up later.  Have fun!

Regards,
Dave


Chris
 

Every ground plane I’ve ever seen has been connected to.. a ground!

The “active” part of the 1/4 wave is usually connected to the center conductor of the coax, the ground plane is connected to the shield and all ground planes are connected together.
That picture is a very wierd combination of a half wave horizontal V dipole and a ground plane. It “may” work but I have my doubts about whether it’s worth the effort as a simple V dipole works quite well.

It doesn’t take much to grab the vhf NOAA sats. I’ve done it with everything from a rubber ducky to a 250 foot long wire.

let’s think of the antenna requirements for low earth orbit. The following assume “normal” reception, not using  specialized (and far more expensive) equipment to squeeze the last second out of each pass.
1) It must be omnidirectional, hear equally well from all directions, because we don’t want to have to aim the antenna at the satellite and track it during each pass.
2) it should have best gain toward horizons and can have less gain directly overhand. The satellite is furthest away and there’s more atmosphere to absorb the signal when the satellite is lower in the sky. Overhead, it’s closer.
3) It should be able to receive both horizontal and vertical polarization. As the satellite passes, the angle of its antenna in relation to the angle of the receive antenna will change.
4) Gain isnt as important as a good “view” to the horizon in all directions. Gain won’t make up for being in the shadow of an object.

in order of complexity, here’s the three common antennas that meet the criteria.

a straight “V” antenna with one element connected to the center conductor and the other to the shield of the coax works quite well as a novice antenna.
pros: really easy to make. Decent performance, moderately omnidirectional in both vertical and horizontal planes, 
cons: changing polarization will cause brief sharp nulls during some phases.

a common vertical ground plane.
pros: can be made out of a panel mount “UHF” connector and a metal coat hanger, completely omnidirectional in the horizontal plane, null directly overhead in vertical plane, pretty good toward the horizon.
Cons: polarization is vertical, expect brief deep nulls as polarization shifts.

a “turnstyle” antenna which is basically two horizontal dipoles as right angles to each other.
pros: omnidirectional, slightly better performance than V dipole
cons: can be difficult to tune and match, changing polarization will cause brief sharp nulls during some phases.

a QFH antenna.
pros: omnidirectional on horizontal and vertical directions, polarization issues are minimal.
cons: mechanically complex, larger, ugly (according to my wife).

The antenna that’s being proposed violates requirement number 2. The ground plane, if configured correctly) will tend to boost signals overhead at the expense of weaker signals toward the horizon. At best, playing with spacing and lengths it could be forced to have a ‘lobe’ toward the horizon, but it would have dead spots as the vertical angle changes. Not a desirable situation for this type of reception. It’s a very strange he hybrid of a V and a ground plane. As a V is normally a dipole with both elements driven, it’s hard to see what the connection to the ground plane should be.

If your starting out, keep it simple, get the antenna outdoors with as large a view of the sky as possible. You will be surprised at how good the pictures are.
if you decide to get serious about this mode, I would jump straight to a QFH and not mess about with small signal improvements for lots of work.

Don’t get bent out of shape about “proper” coax. Normal, inexpensive, cable/satellite TV RG6 is just fine in most situations. Same with “LNA’s”. Unless you have a hearing impaired receiver, you should see pictures without and if you do have a deaf receiver, your better off saving your money and put it toward something decent.


Siegfried Jackstien
 

I made a qfh from a wodden support and 2 stiff wires bent into qfh shape
Easy to build
Hidden in a tree so xyl friendly :-)
I have to check what size i used.. But what i sure do remember.. I first built it for 137 but made it smaller for better performance on the weaker ham sats
Noaa birds on 137 are so strong that you can live with some mismatch
Dg9bfc sigi 

Am 22.12.2021 17:57 schrieb Chris <Deach01ca@...>:

Every ground plane I’ve ever seen has been connected to.. a ground!

The “active” part of the 1/4 wave is usually connected to the center conductor of the coax, the ground plane is connected to the shield and all ground planes are connected together.
That picture is a very wierd combination of a half wave horizontal V dipole and a ground plane. It “may” work but I have my doubts about whether it’s worth the effort as a simple V dipole works quite well.

It doesn’t take much to grab the vhf NOAA sats. I’ve done it with everything from a rubber ducky to a 250 foot long wire.

let’s think of the antenna requirements for low earth orbit. The following assume “normal” reception, not using  specialized (and far more expensive) equipment to squeeze the last second out of each pass.
1) It must be omnidirectional, hear equally well from all directions, because we don’t want to have to aim the antenna at the satellite and track it during each pass.
2) it should have best gain toward horizons and can have less gain directly overhand. The satellite is furthest away and there’s more atmosphere to absorb the signal when the satellite is lower in the sky. Overhead, it’s closer.
3) It should be able to receive both horizontal and vertical polarization. As the satellite passes, the angle of its antenna in relation to the angle of the receive antenna will change.
4) Gain isnt as important as a good “view” to the horizon in all directions. Gain won’t make up for being in the shadow of an object.

in order of complexity, here’s the three common antennas that meet the criteria.

a straight “V” antenna with one element connected to the center conductor and the other to the shield of the coax works quite well as a novice antenna.
pros: really easy to make. Decent performance, moderately omnidirectional in both vertical and horizontal planes, 
cons: changing polarization will cause brief sharp nulls during some phases.

a common vertical ground plane.
pros: can be made out of a panel mount “UHF” connector and a metal coat hanger, completely omnidirectional in the horizontal plane, null directly overhead in vertical plane, pretty good toward the horizon.
Cons: polarization is vertical, expect brief deep nulls as polarization shifts.

a “turnstyle” antenna which is basically two horizontal dipoles as right angles to each other.
pros: omnidirectional, slightly better performance than V dipole
cons: can be difficult to tune and match, changing polarization will cause brief sharp nulls during some phases.

a QFH antenna.
pros: omnidirectional on horizontal and vertical directions, polarization issues are minimal.
cons: mechanically complex, larger, ugly (according to my wife).

The antenna that’s being proposed violates requirement number 2. The ground plane, if configured correctly) will tend to boost signals overhead at the expense of weaker signals toward the horizon. At best, playing with spacing and lengths it could be forced to have a ‘lobe’ toward the horizon, but it would have dead spots as the vertical angle changes. Not a desirable situation for this type of reception. It’s a very strange he hybrid of a V and a ground plane. As a V is normally a dipole with both elements driven, it’s hard to see what the connection to the ground plane should be.

If your starting out, keep it simple, get the antenna outdoors with as large a view of the sky as possible. You will be surprised at how good the pictures are.
if you decide to get serious about this mode, I would jump straight to a QFH and not mess about with small signal improvements for lots of work.

Don’t get bent out of shape about “proper” coax. Normal, inexpensive, cable/satellite TV RG6 is just fine in most situations. Same with “LNA’s”. Unless you have a hearing impaired receiver, you should see pictures without and if you do have a deaf receiver, your better off saving your money and put it toward something decent.



Patrick
 

Would a similar V antenna, adjusted to the proper length, would be suitable for ADS-B or even plane scatter (air scatter) ?


David J Taylor
 

On 23/12/2021 12:38, Patrick wrote:
Would a similar V antenna, adjusted to the proper length, would be suitable for
ADS-B or even plane scatter (air scatter) ?
A collinear made from co-ax would be a better bet for 1.09 GHz ADS-B as it has
more gain. For example:

https://www.balarad.net/

For aircraft scatter use an antenna appropriate to the frequency you wish to
receive.

Cheers,
David
--
SatSignal Software - Quality software for you
Web: https://www.satsignal.eu
Email: david-taylor@blueyonder.co.uk
Twitter: @gm8arv


David Slipper
 

Actually running some tests on the horizontal "V" with a reflector worked rather well.

I found that just the horizontal "V" about 25 feet above the soil had deep nulls at certain elevations but adding the reflector at about 50cm below the "V" got rid of these nulls almost completely and improved the signal strength at low elevations.

I'm rather pleased with the outcome.
Dave


On 22/12/2021 16:57, Chris wrote:

Every ground plane I’ve ever seen has been connected to.. a ground!

The “active” part of the 1/4 wave is usually connected to the center conductor of the coax, the ground plane is connected to the shield and all ground planes are connected together.
That picture is a very wierd combination of a half wave horizontal V dipole and a ground plane. It “may” work but I have my doubts about whether it’s worth the effort as a simple V dipole works quite well.

It doesn’t take much to grab the vhf NOAA sats. I’ve done it with everything from a rubber ducky to a 250 foot long wire.

let’s think of the antenna requirements for low earth orbit. The following assume “normal” reception, not using  specialized (and far more expensive) equipment to squeeze the last second out of each pass.
1) It must be omnidirectional, hear equally well from all directions, because we don’t want to have to aim the antenna at the satellite and track it during each pass.
2) it should have best gain toward horizons and can have less gain directly overhand. The satellite is furthest away and there’s more atmosphere to absorb the signal when the satellite is lower in the sky. Overhead, it’s closer.
3) It should be able to receive both horizontal and vertical polarization. As the satellite passes, the angle of its antenna in relation to the angle of the receive antenna will change.
4) Gain isnt as important as a good “view” to the horizon in all directions. Gain won’t make up for being in the shadow of an object.

in order of complexity, here’s the three common antennas that meet the criteria.

a straight “V” antenna with one element connected to the center conductor and the other to the shield of the coax works quite well as a novice antenna.
pros: really easy to make. Decent performance, moderately omnidirectional in both vertical and horizontal planes, 
cons: changing polarization will cause brief sharp nulls during some phases.

a common vertical ground plane.
pros: can be made out of a panel mount “UHF” connector and a metal coat hanger, completely omnidirectional in the horizontal plane, null directly overhead in vertical plane, pretty good toward the horizon.
Cons: polarization is vertical, expect brief deep nulls as polarization shifts.

a “turnstyle” antenna which is basically two horizontal dipoles as right angles to each other.
pros: omnidirectional, slightly better performance than V dipole
cons: can be difficult to tune and match, changing polarization will cause brief sharp nulls during some phases.

a QFH antenna.
pros: omnidirectional on horizontal and vertical directions, polarization issues are minimal.
cons: mechanically complex, larger, ugly (according to my wife).

The antenna that’s being proposed violates requirement number 2. The ground plane, if configured correctly) will tend to boost signals overhead at the expense of weaker signals toward the horizon. At best, playing with spacing and lengths it could be forced to have a ‘lobe’ toward the horizon, but it would have dead spots as the vertical angle changes. Not a desirable situation for this type of reception. It’s a very strange he hybrid of a V and a ground plane. As a V is normally a dipole with both elements driven, it’s hard to see what the connection to the ground plane should be.

If your starting out, keep it simple, get the antenna outdoors with as large a view of the sky as possible. You will be surprised at how good the pictures are.
if you decide to get serious about this mode, I would jump straight to a QFH and not mess about with small signal improvements for lots of work.

Don’t get bent out of shape about “proper” coax. Normal, inexpensive, cable/satellite TV RG6 is just fine in most situations. Same with “LNA’s”. Unless you have a hearing impaired receiver, you should see pictures without and if you do have a deaf receiver, your better off saving your money and put it toward something decent.


George Stein - NJ3H
 

Hi Dave, 

Do you have any pictures and exact dimensions? Congrats on your receiving success! 

Regards, 
George, NJ3H 
Redmond, Oregon USA 

On Tue, Jan 11, 2022 at 10:31 AM, David Slipper
<softfoot@...> wrote:

Actually running some tests on the horizontal "V" with a reflector worked rather well.

I found that just the horizontal "V" about 25 feet above the soil had deep nulls at certain elevations but adding the reflector at about 50cm below the "V" got rid of these nulls almost completely and improved the signal strength at low elevations.

I'm rather pleased with the outcome.
Dave


On 22/12/2021 16:57, Chris wrote:

Every ground plane I’ve ever seen has been connected to.. a ground!

The “active” part of the 1/4 wave is usually connected to the center conductor of the coax, the ground plane is connected to the shield and all ground planes are connected together.
That picture is a very wierd combination of a half wave horizontal V dipole and a ground plane. It “may” work but I have my doubts about whether it’s worth the effort as a simple V dipole works quite well.

It doesn’t take much to grab the vhf NOAA sats. I’ve done it with everything from a rubber ducky to a 250 foot long wire.

let’s think of the antenna requirements for low earth orbit. The following assume “normal” reception, not using  specialized (and far more expensive) equipment to squeeze the last second out of each pass.
1) It must be omnidirectional, hear equally well from all directions, because we don’t want to have to aim the antenna at the satellite and track it during each pass.
2) it should have best gain toward horizons and can have less gain directly overhand. The satellite is furthest away and there’s more atmosphere to absorb the signal when the satellite is lower in the sky. Overhead, it’s closer.
3) It should be able to receive both horizontal and vertical polarization. As the satellite passes, the angle of its antenna in relation to the angle of the receive antenna will change.
4) Gain isnt as important as a good “view” to the horizon in all directions. Gain won’t make up for being in the shadow of an object.

in order of complexity, here’s the three common antennas that meet the criteria.

a straight “V” antenna with one element connected to the center conductor and the other to the shield of the coax works quite well as a novice antenna.
pros: really easy to make. Decent performance, moderately omnidirectional in both vertical and horizontal planes, 
cons: changing polarization will cause brief sharp nulls during some phases.

a common vertical ground plane.
pros: can be made out of a panel mount “UHF” connector and a metal coat hanger, completely omnidirectional in the horizontal plane, null directly overhead in vertical plane, pretty good toward the horizon.
Cons: polarization is vertical, expect brief deep nulls as polarization shifts.

a “turnstyle” antenna which is basically two horizontal dipoles as right angles to each other.
pros: omnidirectional, slightly better performance than V dipole
cons: can be difficult to tune and match, changing polarization will cause brief sharp nulls during some phases.

a QFH antenna.
pros: omnidirectional on horizontal and vertical directions, polarization issues are minimal.
cons: mechanically complex, larger, ugly (according to my wife).

The antenna that’s being proposed violates requirement number 2. The ground plane, if configured correctly) will tend to boost signals overhead at the expense of weaker signals toward the horizon. At best, playing with spacing and lengths it could be forced to have a ‘lobe’ toward the horizon, but it would have dead spots as the vertical angle changes. Not a desirable situation for this type of reception. It’s a very strange he hybrid of a V and a ground plane. As a V is normally a dipole with both elements driven, it’s hard to see what the connection to the ground plane should be.

If your starting out, keep it simple, get the antenna outdoors with as large a view of the sky as possible. You will be surprised at how good the pictures are.
if you decide to get serious about this mode, I would jump straight to a QFH and not mess about with small signal improvements for lots of work.

Don’t get bent out of shape about “proper” coax. Normal, inexpensive, cable/satellite TV RG6 is just fine in most situations. Same with “LNA’s”. Unless you have a hearing impaired receiver, you should see pictures without and if you do have a deaf receiver, your better off saving your money and put it toward something decent.


Ken Sejkora
 

Hmmm…  I wonder if the “magic” spacing between the horizontal “Vee” antenna and horizontal ‘ground plane reflector’ beneath it might be ¼-wavelength of the frequency.  At 137 MHz, with an open-space wavelength of 2.19 meters, a quarter wavelength would be 54.7 cm.

 

Perhaps someone more adept with 4NEC2, EZNEC, or NEC2 could model the situation and arrive at the optimal configuration for spacing between the “Vee” antenna and underlying ‘ground plane reflector’.  I haven’t mastered 4NEC2, so I can’t offer a more definitive answer.

 

By the way – as of 01-Jan-2022 the author of EZNEC Pro/2 Ver.6 has made the program freely available for download.  If you’re interested in trying antenna modelling, this might be worth considering.  Details can be found here:

 

https://www.eznec.com/

 

Ken  --  WBØOCV

East Falmouth, MA

 

From: George Stein - NJ3H via groups.io
Sent: Tuesday, January 11, 2022 01:57 PM
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] 137MHz "V" Antenna advice needed ...

 

Hi Dave, 

 

Do you have any pictures and exact dimensions? Congrats on your receiving success! 

 

Regards, 

George, NJ3H 
Redmond, Oregon USA 

 

On Tue, Jan 11, 2022 at 10:31 AM, David Slipper

<softfoot@...> wrote:

Actually running some tests on the horizontal "V" with a reflector worked rather well.

I found that just the horizontal "V" about 25 feet above the soil had deep nulls at certain elevations but adding the reflector at about 50cm below the "V" got rid of these nulls almost completely and improved the signal strength at low elevations.

I'm rather pleased with the outcome.
Dave

 


D R
 

Thanks for the follow up feedback, Dave: it's nice to see that a bit of experimentation can bring good results!  Thirty years ago I was doing wxsat imaging with a turnstile antenna fitted with an eight leg reflector, and using a five element Yagi on a rotator to get full H to H reception (even over the horizon when atmospheric conditions were right), but time moved on and changes in computer technology rendered my hardware obsolete, and my antennas were taken down a long time ago.  However, in these days of SDR radios and SDRC, satellite reception is not the black art that it once was, and I think I really ought to give it another go.  I still have the reflector - made from eight lengths of aluminium tube soldered to a piece of copper clad pcb - so the V-dipole would be an easy way back in to start reception again.  I'll see what I can rustle up in the Spring :).

Regards,
Dave


Chris
 

I’m trying to get my head around how a ground plane would help low angle and reduce nulls.
Theoretically, it should have the opposite effect.
That is a really strange design but if it’s working for you who cares :)

My experience with the V dipoles is that they aren’t immune to polarization shifts. It’s quite possible that you weren’t seeing nulls in the pattern as much as nulls due to polarization changes as the bird rotates in relation to the fixed ground station.
Adding the strange reflector may be providing a fluke bounce back to the V that helps with the nulls.
At these frequencies, the height of the antenna above ground (as long as it’s more than a few wavelengths) should have negligible affect on signal strength as long as the antenna has line of sight to the transmitter.

Did you end up connecting the reflector to the coax shield?
Is it grounded in any other way (clamped to the support)?

The 137MHz noaa is quite powerful and it’s easy to overthink the antenna. I’ve had excellent results from almost anything I’ve used. From a 250’ piece of electric fence (no balun or anything to restrict bandwidth) 5’ off the ground to my 144/440 commercial ham radio antenna.

In my experience, there have only been two factors affecting reception of 137MHz birds.
1) polarization shifts causing sharp nulls during the pass
2) Line of sight being blocked by trees, buildings, other antennas, etc.

The QFH is known to be the best compromise, simply because it’s relatively immune to polarization changes and is still also pretty omnidirectional.

You do NOT really want an antenna with gain in the vertical direction. That’s when the bird is closest so it’s just wasted.

Gain comes from somewhere… it’s not magic.
if you have gain in any given direction it’s at the expense of gain in another direction. Take a balloon and squeeze it into a donut. You’ve just made a vertical dipole. Now push in one side (you will need three hands) you’ve just made a yagi, assuming you didn’t pop the balloon.

My first weather sat receiver was a modified broadcast FM tuner, an oscilloscope for a display with a Polaroid camera to capture the image and a reel to reel tape recorder to keep repeating the pass till I got the scope tweaked for any kind of image. I did use a multi element yagi on that as it was quite deaf so I spent much time on the roof manually tracking the bird.
I’m sure there are school safety regulations now that wouldn’t let it happen! Sure wish I had some of those recordings or Polaroids now!

Have fun, experiment and whether it works or not, try to figure out why.
Christopher


Richard Fairman