Re: Build Experimental Loop for LF to HF.


Allan Isaacs
 

A few comments to all that responded.

There are not many varicap diodes with enough swing to tune the VLF band. Using two back-to-back 1SV149 diodes would give me only 250pF so I’d need 8 to arrive at 1000pF from my two or 12 for my triple varicaps of 1500pF.

 

I did discover one rather useful side effect which was given enough amplification the thing would burst into oscillation (what used to be reaction in a TRF receiver) and just before this happened tuning selectivity increased dramatically as did the amplitude of a received signal. It will be fairly simple to add another FET across the buffer amplifier emitter resistor and use the spare wire in my cable to control reaction.

 

The main aim of the ferrite aerial over the loop was to reduce the physical size and to reduce the parasitic capacitance of the coils to give me a greater tuning range. The reason for trying both designs was to eliminate really bad local noise and another, particularly the new ferrite aerial is to help pinpoint the origin of local noise.

Allan G3PIY


From: main@SDR-Radio.groups.io [mailto:main@SDR-Radio.groups.io] On Behalf Of jdow
Sent: 06 December 2020 23:05
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] Build Experimental Loop for LF to HF.

 

Another fun antenna to play with is the dielectric loaded flat spiral if you want to play at UHF for a REALLY broadband efficient antenna.

I took an antennas class in college. The professor was working on those things for use as patch antennas on aircraft to avoid extra drag.

{^_-}

On 20201206 14:38:21, Michael.2E0IHW via groups.io wrote:

A big loop is probably best for RX performance, but has to be, well, BIG -  and robustly built.
The Maynard derivatives, especially the ultralight DX series (search uldx io-group)  perform impressively,
but are heavy and, needing many ferrite rods,  expensive to build.

You takes yer choice...

Michael 2E0IHW

On 06/12/2020 17:59, jdow wrote:

My working levels came from a DoD receiver specification that was lying around. Take two signals into the receiver front end and vary their identical levels upwards until the 3rd order IMD products were 1uv - if I recall correctly. The test signal level in dBm plus 107 dB is the dynamic range. They were looking for 80 dB. (None of that ARRL horse manure about the sum of the levels - each signal was -27 dBm with IMD products at -127 dBm. So I was testing around -20 dBm to -30 dBm.

(And nothing really beats physical size for antennas. And a simple loop can easily be large enough that you need a LOT of ferrite to get equivalent performance. HOWEVER, there is the detail that if the antenna is 0.1% efficient at MW a bigger antenna will generally not give you better performance.)

{^_^}

On 20201206 09:30:02, Mag loop Simon wrote:

Hi
 
In my honest opinion..
 
Ferrite antennas are really down on sensitivity compared to tuned mag loops. ( tried it.)
 
 
Useful for dfing..but no one uses them for low noise dx Rx antennas..there is a good reason for this ..
 
Ok if one builds a ferrite antenna from LOTS of bars then they can start to be of use..but y? When can do much better with a rx only multi turn loop. ( less cost more Q etc.)
 
 
Re varicap diodes for tuning..been done for years..problem now is finding suitable diodes..and really they make any difference to imds etc with the small voltages developed across loop, ( unless real close to AM broadcast station etc.)
 
Main issue with remote high Q loop antennas is tuning..this can easy be got around with stepper motor..
 
I have alot of experience with loop antennas..I use a LARGE one for 160m tx..
 
Simon g0zen 
 

 

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