LW-MW-SW Relay Tuned 15' Noise Reducing Vertical Antenna

Useful information regarding antennas for SDR products.
vk7jj
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Re: LW-MW-SW Relay Tuned 15' Noise Reducing Vertical Antenna

Post by vk7jj » Fri Jul 12, 2019 4:54 am

Hi Gianfranco, fine on your twin wire, thank you for putting me out of my misery. But then you put me right back into a worse state of misery showing that photo of you in the water during our mid winter! Totally callous.

Fine on your matching considerations and hopefully your ferrites will meet expectations, that would be really good. I wonder if you have any other antenna available where you are or if you put the pallet to use because you had no other? A comparison with a known antenna would be interesting.

I hope you're still itching to build a decent sized loop when you get home and you can put that up against your low noise Beverage antenna.

Phil

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glovisol
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Re: LW-MW-SW Relay Tuned 15' Noise Reducing Vertical Antenna

Post by glovisol » Fri Jul 12, 2019 1:37 pm

Hi Phil, I slept on it and was even more convinced this morning, so here is my idea, which is probably the re-invention of the wheel, or perhaps there will be 999 reasons why it will not work. I only have to wait until I get back home in August and test it.

METHOD FOR PASSIVE BROADBAND MATCHING OF A SMALL LOOP ANTENNAS IN HF

This method is based on original work by Philip R. Geffe (Simplified Modern Filter Design) who, as far as I know, was the first (in the fifties) to publish the idea that filter networks can be used not only for frequency band limiting, but also for broadband impedance matching. In his book Geffe gives tabulations for normalized elements of filter neworks working between different input / output impedances. Far more complete tables have been published later by Saal and by Zverev. For complete reference information, look here:

https://www.sdrplay.com/community/viewt ... 026#p16026

Looking at the equivalent circuit of a small single turn loop, we see that it is represented by a resistor of a very low, sub-Ohm value, in series with an inductor. If we consider an useful frequency band extending from 3 to 15 MHz, we see that the median values are R=0.56 Ohm and L=4.6 uH and that the spread is relatively small. It must be noted here that I calculated the loop inductance with another independent formula (*) and I found the same value as given by Phil.

Now my idea is very simple:
a) if I take a simple Butterworth or Chebyshev prototype, say N=5 and
b) de-normalize it @ 15 MHz (or any frequency above the reception range I want from my loop) and
c) also de-normalize between 0.56 Ohm input and a convenient higher output impedance and
d) if the first series inductor has a calculated value higher than the antenna inductance,

THEN

e) I can incorporate the loop inductance into the value of the first network inductor and
f) Obtain perfect wideband match between 3 and 15 MHz without resonating the loop.

I shall now test the idea by calculation and if there is a significant number of simple networks that fulfill the conditions described above, I shall show them next.

(*) Inductance L = 0.008 * s * ln(s/l + 0.37942 + (0.333 * l/s)) = 4.29569 uH

Where:
L: inductance in uH
s: side length, cm = 88
l: coil length = wire diameter cm = 0.2

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sdrom33
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Re: LW-MW-SW Relay Tuned 15' Noise Reducing Vertical Antenna

Post by sdrom33 » Sat Jul 13, 2019 6:00 pm

Hi Phil & Gianfranco, it seems to me that this thread, that had to be closed, because it had become "toxic" according to some guys, is by contrast showing a lot of useful information on small antennas. This is a lesson to us all that a good discussion on technical problems and data is only "toxic" for some egos, but can be very useful for most of us. Pse keep up the good work!

Thomas

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glovisol
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Re: LW-MW-SW Relay Tuned 15' Noise Reducing Vertical Antenna

Post by glovisol » Sat Jul 13, 2019 6:42 pm

More reception examples with non-optimized original cross pallet loop antenna.

The peculiar thing, that comes to attention at once, is the low level of backgrund noise: under these conditions, signals at levels of -115 to -120 dBm are easily copied witout problems, even if the S meter shows S1 to S3.
Attachments
1. Loop 3.6 MHz.png
1. Loop 3.6 MHz.png (313.49 KiB) Viewed 716 times
2. Loop 7.15 MHz.png
2. Loop 7.15 MHz.png (321.14 KiB) Viewed 716 times
3. Loop 14 Mhz.png
3. Loop 14 Mhz.png (319.37 KiB) Viewed 716 times

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glovisol
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Re: LW-MW-SW Relay Tuned 15' Noise Reducing Vertical Antenna

Post by glovisol » Sat Jul 13, 2019 6:47 pm

And this evening at 18:45 GMT with good propagation......
Attachments
ScreenHunter 49.png
ScreenHunter 49.png (323.38 KiB) Viewed 677 times
ScreenHunter 50.png
ScreenHunter 50.png (323.71 KiB) Viewed 677 times

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glovisol
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Re: LW-MW-SW Relay Tuned 15' Noise Reducing Vertical Antenna

Post by glovisol » Sun Jul 14, 2019 5:09 pm

Hi Phil,

You have really convinced me and surely I am going to put up a big loop antenna once I get home in August. Here at the sea side I did not have any antenna at all and fortunately followed your suggestion and started experimenting with small loops. The results are incredible, because I see plenty of signals in the ham bands from 3.65 to 14.3 Mhz, while I have seldom experienced such remarkably low noise levels. The result is that I start copying a signal and then I jump on the chair when I realise I am copying quietly and peacefully a signal at a level of -120 dBm!!

Before the discovery of the loop, I used to set up a "long wire" about 15 m long and tie the return to a water pipe. Then thru the low noise transformer to the HI Z input of the SDRduo. It goes without saying I could copy shortwave broadcast stations, but nothing else. It was a great evening when I could get a ham on 40 m. Signals were weak and noise levels in the order of -100 / -110 dBm. Now with the loop we are below -120 dBm and better, depending on the frequency.

Today I have "optimized" the efficiency of the "space taken" by the antenna (plenty of pallet wood on my hands): I have built a square around the cross, so the side has gone from 88 cm to 129 cm and I shall see the improvement this evening.

Hope to get the toroids tomorrow.

Kind regards,

Gianfranco

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glovisol
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Re: LW-MW-SW Relay Tuned 15' Noise Reducing Vertical Antenna

Post by glovisol » Sun Jul 14, 2019 7:27 pm

Hi Thomas.

I agree with you, even if we should understand that words perhaps written in anger are not really meant and should be quickly forgotten. But you are right in observing that this thread, born under such unhappy auspices, has turned to the better. Now, thanks to Phil's suggestions and help, I know from my own experience, and perhaps a lot of people reading this thread do too, that if one lives in a small apartment, like I am doing these days, he can, in a short time and with little effort, cook up an antenna that will allow him to operate an RSP Processor with reasonable results, spending a few cents of insulated electrical wire and wood self tapping screws (provided he can safely steal a pallet). I have now modified the antenna obtaining a significantly larger aperture area and shall post results next.

I intend to more deeply investigate this small antenna solution in the next few days and seriously test the wideband matching method later on, when I get back and have my lab facilities in hand.

Kind regards,

Gianfranco

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vk7jj
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Re: LW-MW-SW Relay Tuned 15' Noise Reducing Vertical Antenna

Post by vk7jj » Mon Jul 15, 2019 10:13 am

Let's get back to antennas, wonderful things that they are.

Yes, I bet you can't wait to have a play with longer lengths of wire at home.

Even if we all lived in a perfect noise free environment there is clearly a point where the desired signal voltage from a small antenna falls below the level where whatever RF gain one has is still not enough to raise the wanted signal above the noise floor of the receiver.

Put another way, it's clear that many people have limited space but do they really *have* to go out and buy the ubiquitous physically small loop or mini-whip?

I absolutely love trying to give people a bit of understanding of the issues and encourage them to have a go at building something for themselves, as large as their living space will permit. At the end of the day, if it is to be a small loop then try the hula hoop!

https://www.sdrplay.com/community/searc ... words=hula

Large or small, these loops are simple antennas with simple feed systems that are very inexpensive and can so easily be whizzed up if people feel it is worthwhile. That's why I value your efforts so much, what you are doing empowers people.

Phil

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glovisol
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Re: LW-MW-SW Relay Tuned 15' Noise Reducing Vertical Antenna

Post by glovisol » Mon Jul 15, 2019 2:41 pm

Hi Phil, it is exactly that. You pushed me and I am now going full blast.

Yesterday I increased the side of the square from 88 cm to 1.29 cm. The practical footprint space actually taken by the antenna is the same, but the capture surface has more than doubled, from 0.77 sq. metres to 1.66 sq. metres. Using the original "small" antenna in the last few days, it was evident that performance became better and better going up from 3.65 MHz to 14.3 MHz.

Now, with increased dimensions, performance improved slightly at 14.3 MHz, but had a very significant improvement at 3.65 MHz, as proven by the screen enclosed below. No toroids yet, but I keep looking forward to them.

Rather than hula hops, I am thinking of an all stainless steel (or copper) square made with tube shafts tapped at the ends and screwed into machined corner ends. This would be the perfect portable antenna: you carry it in a golf bag and you deploy it anywhere, or you keep it at home and assemble it when needed. I think we are making quite a progress with respect to whips (!) or amplified printed circuit boards. If the new wideband matching works as well, then we shall have a bomb. Please advise me on possible flaws in this scheme.

Of coure we are covered down to 3.65 MHz only, and at the limit there, but lower than that we come into the domain of "large" ferrite antennas and this is a new world to explore. Luckily a fantastic amount of information is available here, even though large ferrite sticks are quite expensive.


Kind regards,

Gianfranco
Attachments
13.7. Loop 14 MHz 2.png
Not bad for a small loop antenna!
13.7. Loop 14 MHz 2.png (314.13 KiB) Viewed 529 times
14.7. 3.65 MHz improved.png
Very big improvement at 3.65 MHz!
14.7. 3.65 MHz improved.png (316.09 KiB) Viewed 533 times
129 cm loop.JPG
Should we call it the square pallet antenna?
129 cm loop.JPG (178.18 KiB) Viewed 533 times

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vk7jj
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Re: LW-MW-SW Relay Tuned 15' Noise Reducing Vertical Antenna

Post by vk7jj » Tue Jul 16, 2019 12:39 am

glovisol wrote:I am thinking of an all stainless steel (or copper) square
A quick note on stainless, here is a pic of a full size 160m delta using copper, aluminium and steel wire.
Copper is line 1, aluminium line 2 and steel line 3. Wire radius is 0.8mm in all cases.
Screen Shot 2019-07-16 at 10.36.29 am.png
Screen Shot 2019-07-16 at 10.36.29 am.png (50.89 KiB) Viewed 497 times
Look at the R (ohms) and weep! Unless you want a great SWR on transmit and don't mind keeping the sparrows feet warm!

Best regards, Phil

Edit:

To clarify, R (Ohms) is the sum of the antenna's radiation resistance and the wire's inherent resistive loss.
The figures given are at the fundamental frequency of the loop which is 1.8MHz, chosen because it provides a low radiation resistance and thus makes the wire loss all the more obvious as it is a larger percentage of the total resistance, ie. 163 ohms of resistive loss in the wire compared with a radiation resistance of around 20 ohms.

Small loop R as we know is a fraction of an ohm at or below their fundamental frequencies and lots of efforts goes into making the resistive losses of so called magnetic loops as low as possible.

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