Extending the range of a Discone antenna down to 3.6 MHz

Useful information regarding antennas for SDR products.
glovisol
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Location: Piedmont, Italy

Extending the range of a Discone antenna down to 3.6 MHz

Postby glovisol » Tue Feb 05, 2019 4:58 pm

Two weeks ago I installed a Comtrack Discone antenna model D-130, rated 25 to 1300 MHz. I was curious to compare it to the 135 m Beverage and to see the performance in the (not rated!) bands 80 /40 & 20 meters.

Go to next post for text amendment purposes.

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glovisol
Posts: 493
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

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

Postby glovisol » Tue Feb 05, 2019 5:12 pm

Antenna is mounted on a 1.8 m pole on the roof and comes to the RSPduo via a 20 m length of RG 58 coax cable (50 Ohm). What I immeditely noticed was that the Discone was 5 to 10 dB noisier than the Beverage and this kept me thinking all the time. Furthermore I could see a hint of signal coming in at 3.6 and 7 Mhz., but buried in the noise.

Then the idea came: why not try a 50 to 50 Ohm, low capacitance, isolating balun? Presto! It was done in five minutes and the result on 3.6 MHz is shown below: quite amazing! The balun is also a good idea in protecting the RSPduo coaxial input. There is no connection to any ground anywhere, save on the antenna mast.

Balun data is as follows:

Toroid core: FT-37-43 (material type 43)
Ext. dia.: 0.375"
Internal dia.: 0.187"
Thickness: 0.125"
PRIMARY = SECONDARY: 5 turns enamel copper wire 0.5 mm dia.

Performance at 7 MHz is similar. Have yet to try on 14 MHz, due to very bad propagation.

Of course at 144 MHz and at air band the antenna is stars....without any balun.
Attachments
Discone balun.JPG
Low capacitance 50 - 50 Ohm isolating balun
Discone balun.JPG (76.63 KiB) Viewed 2333 times
Discone 1 - H. 17_30.jpg
Plenty of signals above noise with isolation
Discone 1 - H. 17_30.jpg (197.54 KiB) Viewed 2333 times
Discone 2 - H.17_31.jpg
Poor show with antenna straight thru to the RSPduo
Discone 2 - H.17_31.jpg (186.09 KiB) Viewed 2333 times

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glovisol
Posts: 493
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

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

Postby glovisol » Tue Feb 05, 2019 7:03 pm

Under the propagation conditions shown in previous PC screens, the difference in level between Beverage abd Discone was 10 dB approximately. What I notice now (one hour later) is that from time to time the Discone experiences severe fading, while the signal on the Beverage stays relatively constant and readable.
Attachments
Discone under fading.jpg
Discone under fading.jpg (190.4 KiB) Viewed 2314 times
Beverage in same conditions.jpg
Beverage in same conditions.jpg (193.96 KiB) Viewed 2314 times

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JMG
Posts: 23
Joined: Thu Mar 26, 2015 9:58 pm

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

Postby JMG » Tue Feb 05, 2019 9:29 pm

This has been reported many times before.

It even works down below 10kHz. So, a well disgned installation will give you a frequency range of 10kHz to 2Ghz - at full sensitivity (SNR)!

Background: at low frequencies the antenna cable (or the antenna pole) acts as a grounded vertical stub antenna. The discone antenna elements represent some capacitance to ground.
Some problems will arise at frequencies where the length of the antenna cable comes close to half a wavelength.

A balun can not be recommended.

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glovisol
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Location: Piedmont, Italy

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

Postby glovisol » Tue Feb 05, 2019 10:54 pm

It would be very helpful of you to upload some references on the Discone antenna parasitic behaviour you mention: this information could be of help to a lot of users.

However this does not seem to be the case, as would have been evident if I had uploaded not a close-up, but a wide angle photo showing antenna cable parasitic insulation by means of ferrite suppressors, as shown below.
Attachments
Discone insulated feed.JPG
Discone insulated feed.JPG (93.14 KiB) Viewed 2282 times

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illllm
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Joined: Wed Jan 23, 2019 10:26 pm

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

Postby illllm » Wed Feb 06, 2019 8:19 pm

I tried a 1:1 to isolate my RSp2 and it actually increased the noise floor. :o I wonder what I am doing wrong.

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JMG
Posts: 23
Joined: Thu Mar 26, 2015 9:58 pm

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

Postby JMG » Wed Feb 06, 2019 9:51 pm

An excelent presentation of the fundamentals of the discone antenna, mounted on a pole, can be found here:
https://www.youtube.com/watch?v=vFYVpJ9FPZ8

Please look at 12:20.
Sory, it is in German.

If you want to run some simulations: the total capacitance of the elements of the DJ-130 antenna is 25pF.

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ON5HB
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Location: Heppen - Belgium
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Re: Extending the range of a Discone antenna down to 3.6 MHz

Postby ON5HB » Thu Feb 07, 2019 8:39 am

glovisol wrote:Antenna is mounted on a 1.8 m pole on the roof and comes to the RSPduo via a 20 m length of RG 58 coax cable (50 Ohm). What I immeditely noticed was that the Discone was 5 to 10 dB noisier than the Beverage and this kept me thinking all the time. Furthermore I could see a hint of signal coming in at 3.6 and 7 Mhz., but buried in the noise.

Then the idea came: why not try a 50 to 50 Ohm, low capacitance, isolating balun? Presto! It was done in five minutes and the result on 3.6 MHz is shown below: quite amazing! The balun is also a good idea in protecting the RSPduo coaxial input. There is no connection to any ground anywhere, save on the antenna mast.


What you have done is disconnect the common-mode-currents from the coax, but you did it in the shack.
If you do it at the discone the coax will stop receiving signals.
It may stop working on very low bands in total but the noise will go down also.

I suggest you try a common-mode-choke with a plastic bottle at the antenna, see what happens as the coax then stops receiving signals in total.

Very good article about it can be found here: http://www.karinya.net/g3txq/chokes/

Ps. doing it at both ends is no problem either. But to block noise from the coax it's normally done at the antenna side.

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Greetings,

Bas - ON5HB

glovisol
Posts: 493
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

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

Postby glovisol » Thu Feb 07, 2019 2:34 pm

Thank you Bas!! I am going to stick a bottle choke on the roof right away & report results...

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ON5HB
Posts: 113
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Location: Heppen - Belgium
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Re: Extending the range of a Discone antenna down to 3.6 MHz

Postby ON5HB » Thu Feb 07, 2019 3:10 pm

Here is an idea on how to construct such.

http://www.hamblog.co.uk/constructing-an-ugly-balun/

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Greetings,

Bas - ON5HB


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