Frame antenna query

Hi Andy,
I know it is a bit late now, but I see that there was a question here that was hidden and not answered.

That which I do for matching loop and frame antennas is estimate the capacitance needed to bring the antenna to resonance, from this I get the loop impedance.
From this I can calculate (simple transformer principle) the ratio to match to 50Ω.

If the loop antenna is used for a wide frequency range, then the feed impedance will vary with frequency, but as long as it is resistive and in the right general area then it should still work very well.

INFO - If you have trouble estimating the capacitance, then use a signal generator and check the resonant frequency. Add an extra capacitor at the end of the loop and select a value where the frequency falls by 35%. The test capacitor is the same value as the loop capacitor.

Is this info useful?

Hi Harry,

that information is very useful, thank you very much.. I have recently bought a nano-VNA and will continue experimenting

Many thanks
Andy

G7MJV wrote:Many thanks for letting me join, I'm g7mjv from the UK

I've recently constructed a 20M frame antenna and I'm using it for receiving FT-8 with an RTL-SDR.

My query is the formula for working out the size of the pick-up loop. Am I right in thinking that it should be 0.03 of a wavelength?

It could be that my maths is wrong but here we are.. based on 14.175MHz, a wave length is 21.15 meters. That times 0.03 gives 0.6345 M

It's receiving ok, given the conditions, but it just looks small compared with some of the photos that I've seen.

Many thanks, Andy

Hi Andy,
I know it is a bit late now, but I see that there was a question here that was hidden and not answered.

That which I do for matching loop and frame antennas is estimate the capacitance needed to bring the antenna to resonance, from this I get the loop impedance.
From this I can calculate (simple transformer principle) the ratio to match to 50Ω.

If the loop antenna is used for a wide frequency range, then the feed impedance will vary with frequency, but as long as it is resistive and in the right general area then it should still work very well.

INFO - If you have trouble estimating the capacitance, then use a signal generator and check the resonant frequency. Add an extra capacitor at the end of the loop and select a value where the frequency falls by 35%. The test capacitor is the same value as the loop capacitor.

Is this info useful?

G7MJV wrote:Hello Glen,Thank you so much for the information.My 20M loop is a 3 turn one whilst my 10M loop is a 2 turn loop. I have been using both for receiving FT8 and them work very well, I'm impressed. I shall carry on reading about loop antennas and may consider a traditional mag loopKind regardsAndy - G7MJV

Hi Andy

If you need an automatic mag-loop controller, TF3LJ has constructed an open source one - I have built it - it works (I later broke my loop gear though, because of negligence - not the controllers fault). It is to date the only open source automatic mag-loop controller I have found. And it is versatile, because of a lot of options. He sells PCB for it.

My loop controller drove the stepper motors (using/bought DRV8825 through ebay) through maybe 15 meters of cat 5 installation cable - and it works fine, because the stepper motors controller are current driven. Please note that the chosen stepper motors controller do not need heat sinks (small ones can be added - I did), because of good and efficient control. Because of the twisted pair in the cable, it ought not to emit noise:

https://tf3lj.isageek.net/to-automatically-tune-a-magnetic-loop-antenna.html

https://tf3lj.isageek.net/

https://github.com/lofturj/loopController

https://groups.io/g/loopController

Rev02 - 2019-10-13 TF3LJ/VE2AO
Automatic Magnetic Loop Controller
Bill of Materials and Building Instructions
for Printed Circuit Boards Rev3/Rev4 (using Pololu A4988 or DRV8825):
https://tf3lj.isageek.net/to-automatically-tune-a-magnetic-loop-antenna/Automatic_Magnetic_Loop_Contr-2b20828ce9bb9238.pdf

DRV8825 Stepper Motor Driver Carrier, High Current (can e.g. be bought on ebay):
https://www.pololu.com/product/2133
Quote: "...
Stepper motors typically have a step size specification (e.g. 1.8° or 200 steps per revolution), which applies to full steps. A microstepping driver such as the DRV8825 allows higher resolutions by allowing intermediate step locations, which are achieved by energizing the coils with intermediate current levels. For instance, driving a motor in quarter-step mode will give the 200-step-per-revolution motor 800 microsteps per revolution by using four different current levels.
..."

Draft Rev 4 – 2016-04-23 TF3LJ/VE2LJX
Automatic Magnetic Loop Controller
Operating Instructions:
https://tf3lj.isageek.net/to-automatically-tune-a-magnetic-loop-antenna/Automatic_Magnetic_Loop_Contr-b16ac27ee69f037d.pdf
Quote: "...
The Controller can communicate with the following Radios:

Elecraft K3 / KX3
ICOM CI-V (all relatively recent ICOM HF transceivers)
Kenwood TS-440, TS-450
Kenwood TS-870 (not tested yet)
Kenwood TS-480, TS-590, TS-2000
Yaesu FT-100, FT-100D
Yaesu FT747GX (not tested yet)
Yaesu FT-817, FT-847, FT-857, FT-897
Yaesu FT-920 (not tested yet)
Yaesu FT-990
Yaesu FT-1000MP (not tested yet)
Yaesu FT-1000MP MkV
Yaesu FT-450, FT-950, FTdx1200, FT-2000, FT-2000D, FTdx3000, FTdx5000...
TenTec Argo V, Argo VI, Eagle, Omni VII... (these have not been tested yet)
Pseudo VFO (used with non-serial enabled radios)
..."

-

Combine it with this SWR-bridge/coupler/directional wattmeter and you can tune e.g. with 100mW or less. (By adding more secondary turns on all three transformers, it will work with higher powers - I think I used 20 or 24 windings instead of 10):

W7EL, Feb, 1990 issue of QST magazine.
A simple an accurate QRP Directional Wattmeter.
Make a few small enhancements to the Bruene watmeter and diode detector, and you have a directional wattmeter that's simple, portable, and accurate from 10 watts down to 5 milliwatts!:
http://www.qsl.net/kl7jef/QRP%20Directional%20Wattmeter.pdf
backup:
https://web.archive.org/web/20170313085539/http://www.qsl.net/kl7jef/QRP%20Directional%20Wattmeter.pdf

February 4, 2015, More Notes on Directional Couplers for HF -- the Bruene Coupler, Part 1:
https://k6jca.blogspot.com/2015/02/more-notes-on-directional-couplers-for_4.html
backup:
https://web.archive.org/web/20240127135028/https://k6jca.blogspot.com/2015/02/more-notes-on-directional-couplers-for_4.html
Quote: "...
Here's an interesting take on the Bruene Coupler, published by W7EL in the Feb, 1990 issue of QST magazine.
Per the article, this version has +/- 7% accuracy over the range of 1 to 432 MHz. Quite impressive!
...
The only negative that I can see with respect to the design is that you need to wind 3 transformers!
..."

( also posted the SWR info on https://sm0vpo.forumotion.com/t624-mod-swr-to-power-meter-questions )

G7MJV

...
I've recently constructed a 20M frame antenna and I'm using it for receiving FT-8 with an RTL-SDR.

My query is the formula for working out the size of the pick-up loop. Am I right in thinking that it should be 0.03 of a wavelength?
...
Many thanks, Andy

Hi Andy

You are making an one winding loop?

If that is case:

Home Made – HIGH POWER Magnetic Loop Antennas:
https://amrron.com/2015/07/24/home-made-high-power-magnetic-loop-antennas/
backup:
https://web.archive.org/web/20150810143126/https://amrron.com/2015/07/24/home-made-high-power-magnetic-loop-antennas/
Quote: "...
Primarily loop couplng
(Appoximately 1/5 the size of the secondary)
..."

Hello Glen,Thank you so much for the information.My 20M loop is a 3 turn one whilst my 10M loop is a 2 turn loop. I have been using both for receiving FT8 and them work very well, I'm impressed. I shall carry on reading about loop antennas and may consider a traditional mag loopKind regardsAndy - G7MJVGlenndk

G7MJV wrote:
...
I've recently constructed a 20M frame antenna and I'm using it for receiving FT-8 with an RTL-SDR.

My query is the formula for working out the size of the pick-up loop. Am I right in thinking that it should be 0.03 of a wavelength?
...
Many thanks, Andy

Hi Andy

You are making an one winding loop?

If that is case:

Home Made – HIGH POWER Magnetic Loop Antennas:
https://amrron.com/2015/07/24/home-made-high-power-magnetic-loop-antennas/
backup:
https://web.archive.org/web/20150810143126/https://amrron.com/2015/07/24/home-made-high-power-magnetic-loop-antennas/
Quote: "...
Primarily loop couplng
(Appoximately 1/5 the size of the secondary)
..."


An Overview of the Underestimated Magnetic Loop HF Antenna:
http://www.ahars.com.au/uploads/1/3/9/8/13982788/article-antenna-mag-loop-2.pdf
backup:
https://web.archive.org/web/20210322183633/http://www.ahars.com.au/uploads/1/3/9/8/13982788/article-antenna-mag-loop-2.pdf
https://web.archive.org/web/20190713142322/http://www.ahars.com.au/wp-content/uploads/2015/10/The-Underestimated-Magnetic-Loop-HF-Antenna-V1.-2.pdf
Quote: "...
The impedance seen looking into this auxiliary feed loop is determined solely by its diameter with respect to that of the primary tuned resonator loop. A loop diameter ratio of 5:1 typically yields a perfect match over a wide 8:1 or greater frequency range of main loop tuning.
...
The effective feed impedance of the secondary loop is controlled by its diameter / ratio of area and by the number of flux lines cutting it; thus the impedance seen looking into the secondary loop will be essentially independent of frequency.
...
For Rin = 50 Ohm, Dc/Dm = 5.3
In practice coupling reduces when the matching loop is moved away from the tuned loop conductor towards the loop centre. Coupling is also reduced if the match loop wire diameter is reduced.
Typically the frequency invariant parallel equivalent radiation resistance Rp of the resonator loop is circa 40k Ohms for practical loaded Q values in the range 250 to 400. Rin is 50 Ohms.
...
Now not only is the Rp resistance invariant with frequency it is also invariant if the loop size is scaled up or down. The loop inductance is directly proportional to loop size and the radiation resistance increases as the square of the loop size; thus these two variations cancel out in the above equation leaving Rp virtually constant...remarkable.
..."

I will continue to experiment, thank you

Hi Andy,
the size of the pick-up loop affects matching the antenna to the downlead (usually a coax cable) resp. TRX input. You can try one, measure the SWR and tweak the size of the loop accordingly. Unfortunately I have no experience with this type of antenna. A reply by Harry himself would be more precise.

BR from Ivan OK1SIP

Many thanks for letting me join, I'm g7mjv from the UK

I've recently constructed a 20M frame antenna and I'm using it for receiving FT-8 with an RTL-SDR.

My query is the formula for working out the size of the pick-up loop. Am I right in thinking that it should be 0.03 of a wavelength?

It could be that my maths is wrong but here we are.. based on 14.175MHz, a wave length is 21.15 meters. That times 0.03 gives 0.6345 M

It's receiving ok, given the conditions, but it just looks small compared with some of the photos that I've seen.

Many thanks, Andy