Extending the range of a Discone antenna down to 3.6 MHz

Useful information regarding antennas for SDR products.
glovisol
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Re: Extending the range of a Discone antenna down to 3.6 MHz

Post by glovisol » Thu Feb 07, 2019 4:37 pm

Here is the detailed report:

1) The antenna is mounted on a 1.8 m steel pole which goest through the roof and is mounted with two steel brackets to a wooden beam of the roof. There is an elecrical (not RF!)) ground tied to the pole which was wired ages ago and is connected to the general electrical wiring of the house.

2) At the base of the Discone there is an UHF female connector. A male UHF connector connects here carrying the RG58 coax. The coax comes in through the roof with a length of 2.4 m and here I have wound the choke using extra length cable I had prudently left in place. I used a 4" dia. standard water bottle full of water for easy winding, then emptied the bottle & dried the inside with compressed air.

3) The choke amounts to 12 full turns, close spaced and looking at Bas' reference should be good from 2.5 to 30 MHz. I removed the toroids at the receiver's end.

4) Performance is as follows

144 - 148 MHz - no isolation transformer - good performance.
118 - 136 MHz - no isolation transformer - ood performance.
21 MHz - cannot test, no propagation, will report later.
14 MHz - cannot test, no propagation, will report later.
7 MHz - no isolation transformer - good performance: loss compared to 135 m Beverage is 12 to 15 dB.
3.6 MHz : with no isolation transformer no signal gets thru, noise level (power meter) is -94 dBm: antenna not useable.
3.6 MHz : with isolation transformer noise level (power meter) is -112 dBm and signals do get thru: loss compared to 135 m Beverage is 18 to 20 db.

From all the above it would seem that this very small Discone is still useable at 3.6 MHz, provided an isolation transformer is used at the receiver's end. More tests to follow.
Attachments
Discone compare Discone.jpg
Discone compare Discone.jpg (189.88 KiB) Viewed 3346 times
Discone compare Beverage.jpg
Discone compare Beverage.jpg (195.02 KiB) Viewed 3346 times

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ON5HB
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Re: Extending the range of a Discone antenna down to 3.6 MHz

Post by ON5HB » Thu Feb 07, 2019 4:53 pm

One thing you forgot to mention. Does it work better?

Did the noise level go down as I said it would?

For people that have no clue what we are talking about, let me explain in simple words.

Coaxial cable has 3 conductors, what you say????

Yes 3 conductors, let me show you:

1: The inner conductor, that is the core wire of coax.

2: The inner shielding conductor, that is the inside of the shielding of the coax.

3: The outer shielding conductor, what?, yes the outer shielding does also conduct signals!!

What happens is the outer conductor receives signals, they travel up the line into the antenna itself en go into the cable at the inside and get down into your receiver.
That is why coax is often seen as "worse" then open/ladder-line or Hi-Z input. The problem is many people forget to block the outer-shielding currents.
This is called common-mode-currents. You do not want those, not at reception and certainly not when transmitting.

So you need to block them, search for 1:1 balun, poor mans balun or common-mode-chocke....all do the same and they turn coax into a balanced line instead of a 3-wire-unbalanced.

That is the problem when using coax...but they never tell you this :D

Solve this and coax starts working well on HF (and even VHF). :lol:

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glovisol
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Re: Extending the range of a Discone antenna down to 3.6 MHz

Post by glovisol » Thu Feb 07, 2019 5:36 pm

Hi Bas,

Yes, the noise did go down on (almost) all HF frequencies, I thought this would be clear from the PC screens. What I had before was that at 7/14/21/28 Mhz I had noise higher than the Beverage, while signals, when present, were lower than those from the Beverage. Now on 7/14/21/28 noise is lower (as the signal) than that of the Beverage, as it should be.

The exception is at 3.6 MHz. Here the noise was -95 dbm yesterday and still is -95 dbm today (power meter measurement) if I do not use the isolation transformer signals are buried in the noise. As soon as I stick the isolation transformer in, noise drops to -117 dbm (power meter) and the receiver comes to life with plenty signals.... a big result for the cost of a 50 cent toroid!

Following your analogy of noise travelling back and forth, I think the isolation transformer simply, but effectively, completely blocks the noise currents at 3.6 MHz, making the antenna fully useable at this frequency as well.

' 73, glovisol

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Roger
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Re: Extending the range of a Discone antenna down to 3.6 MHz

Post by Roger » Thu Feb 07, 2019 5:47 pm

ON5HB -Bas,

That was a very good description of the "third wire" in coax cable and how it can produce common mode receive and transmit problems. Properly designed chokes, in the right place can result in considerable noise reduction.

Thanks for the posting the link to the G3TXQ site and the common mode chokes info. G3TXQ was well known on many sites as a knowledgeable and considerate ham radio operator. He passed away recently and will be missed by the community.

Roger

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vk3alb
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Re: Extending the range of a Discone antenna down to 3.6 MHz

Post by vk3alb » Thu Feb 07, 2019 7:54 pm

glovisol wrote:Here is the detailed report:

1) The antenna is mounted on a 1.8 m steel pole which goest through the roof and is mounted with two steel brackets to a wooden beam of the roof. There is an elecrical (not RF!)) ground tied to the pole which was wired ages ago and is connected to the general electrical wiring of the house.
This is an interesting point. What happens to your experiment if the ground wire is removed?

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JMG
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Re: Extending the range of a Discone antenna down to 3.6 MHz

Post by JMG » Thu Feb 07, 2019 11:44 pm

Installing a common mode choke at the upper end of the antenna cable, the antenna will not work at low frequencies. At low frequencies the primary antenna element is the antenna cable.
Installing the choke at the lower end results in a completely different antenna: it is a unsymmetrical dipole then, one leg given by the antenna elements of the discone and the other by the antenna cable. The feed point is in between at the discone side.

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glovisol
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Re: Extending the range of a Discone antenna down to 3.6 MHz

Post by glovisol » Fri Feb 08, 2019 7:49 am

1) Hi VK3 ALB, this is a very good idea. I have just removed the grounding wire and now the pole stands on the roof beam fully insulated by the wood. I cannot do testing now (8:35 A.M. - 7:35 GMT) but will report this evening.

2) Hi JMG, your last post, in contradiction with what you apparently specified in your original post (but I might have misunderstood....) and stated by others, especially ON5HB, has brought this matter to utter confusion....If I cannot connect the Discone directly with a 50 Ohm cable because of noise and if I cannot use an RF choke to insulate common mode and furthermore if I cannot insulate and cut the common mode with an in between isolation transformer, how on earth can I connect a Discone antenna to my receiver? I now appeal to your expertise to get from you a precise indication and perhaps a diagram on how my Discone should be connected to my RSPduo.

3) As things stand now I think I am having excellent performance for my Discone, to the point I think of building a larger one for extending the low frequency range. A point I forgot to mention is that (on the RSPduo and at 3.6 MHz)) I normally keep the RF gain at 57 dB (third notch from top with the Bevarage) and at 64.1 dB (full gain) with the Discone.

Kind regards to all,

glovisol

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vk3alb
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Re: Extending the range of a Discone antenna down to 3.6 MHz

Post by vk3alb » Fri Feb 08, 2019 12:45 pm

Hi Glovisol,

Yes it seemed to me that the electrical earth may be acting as an antenna for things other than interesting radio signals. I have seen noise carried by mains wiring cause significant noise on HF. Normal noise reduction techniques using ferrite cores applied to the wiring brought a great improvement to reception. It's possible your unmodified (earthed) system may be suffering from the same thing.

I appreciate your experimentation. You cause us to think rather that accept what we see in front of us. I look forward to your results.
glovisol wrote:1) Hi VK3 ALB, this is a very good idea. I have just removed the grounding wire and now the pole stands on the roof beam fully insulated by the wood. I cannot do testing now (8:35 A.M. - 7:35 GMT) but will report this evening.

Kind regards to all,

glovisol

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Lou
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JMG
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Re: Extending the range of a Discone antenna down to 3.6 MHz

Post by JMG » Fri Feb 08, 2019 12:55 pm

Some time ago I did run some tests and simulations on the antenna at low frequencies. Here you find a comprehensive presentation of the basics. It may help to answer some of the questions.
At my installation (l = 15m) the antenna gives good results up to a frequnecy of a wavelength of 4l=60m or f=5MHz.
The voltage distribution on the cable (sin) shows a minimum at a length h of half a wavelength. Uh will be unusable low.
001.jpg
001.jpg (205.47 KiB) Viewed 3288 times

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glovisol
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Re: Extending the range of a Discone antenna down to 3.6 MHz

Post by glovisol » Fri Feb 08, 2019 6:58 pm

I have removed the electrical ground and found no difference, so the bottom line is that the Discone antenna, plus the support pole, plus the 20 m coax cable "float" with no ground connection at all and provide the already described performance.

The 20 m band opened up and again I had to use the low capacitance isolation transformer for best performance, as shown in the comparison with the Beverage screens below. As soon as I saw that also the 20 m band required the isolation transformer and furthermore saw the concise explanation uploaded by JMG (thanks!) the penny dropped and I drew what happens in the pictorial below.

The Discone + the support pole + the 20 m coax cable form an extra, top capacitance loaded, antenna at 1/4 wavelength (3.6 MHz) and at full wavelength (14 MHz). Local noise travels on the the shield and reaches the RSP receiver at a high impedance. The top mounted coax choke eliminates the top portion, but it is not enough. What kills the remaining noise is the low noise magnetic transformer: primary shorts out the high impedance noise, but true antenna signal current flows thru it and reaches the secondary, providing a "clean" signal to the RSP. For this trick to work, the transformer MUST really be LOW CAPACITANCE, 7 pF or less. I made another transformer with a type 73 material binocular core and all noise gets through, no dice. Will show this in the next post. To conclude, my idea is that all antenna lead-ins for reception must be balanced and therefore the balanced inputs of the RSP's are a great asset, as fantastic assets are low capacitance / low noise transformers on antenna leads, as I have been experiencing since June 2018.

In any case I had no idea a small Discone could perform so well down to 3.6 MHz.
Attachments
Discone phenomenon pictorial.jpg
Discone phenomenon pictorial.jpg (109.19 KiB) Viewed 3269 times
Discone @14 MHz with xfrmr.jpg
Discone @14 MHz with xfrmr.jpg (196.6 KiB) Viewed 3269 times
Beverage @ 14 MHz.jpg
Beverage @ 14 MHz.jpg (196.74 KiB) Viewed 3269 times

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