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

Postby glovisol » Sun Feb 10, 2019 2:58 pm

DISCONE ANTENNA ANALYSIS - IT'S NOT THE DISCONE, IT'S THE WHIP

Discone antenna dimensions are approximately as follows:

Top hat diameter D = 82 cm
Antenna element length Ls = 120 cm
Antenna height Lv = 105 cm

Reverse engineering these dimensions gives an approximate lower cutoff frequency of between 70 and 80 MHz. In effect the antenna pulls in Aircraft Band signals and 2 m Amateur Band signals much better than the internal whip I have on my desk and of course it must be as dead as an old shoe down at 3.6 MHz. But....but on top, above the top hat, the antenna has a whip 1.5 m long above a vertical section obviously housing a trap. This whip acts as a vertical element above the ground plane formed by the Discone's top hat and this is what must pick up (very well, I must say, considering its dimensions) the HF signals.

HI Bas, I see we had the same idea....in any case as a stand by all around antenna, once the noise is properly isolated, this Discone can hold its own!

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

Postby Roger » Sun Feb 10, 2019 4:19 pm

JMG wrote:@glovisol
The discone itself NEVER will receice any reasonable signal at low frequencies! It is much to small compared to the wavelength.


JMG - I agree with you. The receiving element of this antenna is actually the outer shield of the transmission line. Much like the PA0RDT mini-whip (so called e-probe) which picks up the signal on the outer shield as well.

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

Postby glovisol » Sun Feb 10, 2019 6:44 pm

I am glad general consensus has been reached on this matter!

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

Postby glovisol » Mon Feb 11, 2019 8:01 am

Well.......to summarise there are three different theories on the carpet here about the unexpected but remarkable operation of the model D-130 Discone, without forgetting there is a 15 to 20 diB difference with the Beverage:

1) It is caused by the coaxial cable downlead acting as part of a capacitive hat antenna,
OR
2) It is simply the topmounted & trapped vertical whip which picks up signals in the HF band and delivers them to the RSP,
OR
3) it is a mixture of the two operating modes.

It would be most interesting to find out which is the valid theory. This is easy, just go on the roof and remove the whip, but cannot be done now, as I have plenty snow there. This I will do as soon as the roof is clean again and report back

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

Postby ON5HB » Mon Feb 11, 2019 10:03 am

My money is mostly on the coax and some effect from the whip.

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Greetings,

Bas - ON5HB

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

Postby Roger » Sun Feb 17, 2019 6:29 pm

ON5HB wrote:My money is mostly on the coax and some effect from the whip.


Bas,

After all this lengthy discussion you summarised it nicely. The coax is the antenna and the whip is acting as a counterpoise. No magic here!

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

Postby ON5HB » Sun Feb 17, 2019 6:31 pm

Did I win a fridge now? Or am I on to the next round? :lol:

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Greetings,

Bas - ON5HB

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

Postby glovisol » Mon Feb 18, 2019 11:39 am

I have removed the whip from the discone antenna, will check the 3.6 MHz reception this evening & report results tomorrow.

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

Postby glovisol » Mon Feb 18, 2019 2:50 pm

ON5HB wrote:My money is mostly on the coax and some effect from the whip.


Hi Bas,

I really do not understand. If you look at previous posts, you advised me to install a coaxial choke at the base of the Discone antenna, which I did. This choke you said it would block signals picked up by the coaxial downlead: in fact, as I reported to you, the noise went down. Then JMG stated that in this fashion the cable could still pick signals up, but, as shown in previous posts, I also have a choke at the other end (the receiver end) of the cable. So, as far as I have been instructed, theoretically my lead in cable should not pick up any signal at all. Or if it tends to pick it up, it should be greatly attenuated by the chokes. Could you please explain?

I shall do receiving tests this evening, now that I have removed the whip.

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

Postby ON5HB » Mon Feb 18, 2019 4:04 pm

Chokes are like filters, they bring signals down.
But it never blocks 100%.

It just depends how good your chokes are.
Big signals are harder to block then noise.

What they do is make the impedance of the outside of the cable higher than the cable itself, the more impedance you add the better the filter will work.
For transmit purposes a typical filter is between 500 to 5000 Ohm, but as your antenna may be way off in impedance it could be too little and as such still receive.

Such filters are trial and error, very easy to test with transmitters, the more power you have the more simple testing testing is.
As unwanted signals find their way in cheap Chinese computer speakers with no good RF-blocking :lol:

Blocking LF, VLF or even MW is very hard and chokes get big. to give you an idea, my antenna for 160m has 3 meters of ferrite clamped on it to keep 1KW from the outside of the coax. I used 1 meter when running 100W and it worked, it failed with 1KW.

For an receiving antenna it's tricky to build a good one as it's hard to measure, at least I have no idea how.

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Greetings,

Bas - ON5HB


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