Is it possible to tune a discone antenna?

[kamilkamien]

Hi!
I have a Sirio SD1300 antenna connected to a broadband scanner. I usually only listen to the 108-180mHz range. This is the range of aviation, fire brigade, police and all other services in the analog band. I know that the antenna is factory-adapted to the 25-1300mhz range, but I don't need such high frequencies. Is there a way to tune this antenna to the band I use most often?

 

[chief21]

The primary advantage of a discone antenna is its' ability to receive across a wide range of frequencies. Any attempt to "tune" a discone for higher gain would likely require substantial physical re-working of the elements, essentially destroying the discone.
If you're truly only interested in the 108-180 range, I would suggest obtaining a true gain antenna, designed for that particular range of frequencies.

 

[kamilkamien]

Yes, yes, I know the properties of my discone. The simplest solution would be to buy one in the range I am writing about, although I was wondering if it is possible to shorten the radiators from mine to the appropriate length and adjust them to the range I am interested in.

 

[ArloG]

Probably should start here:

Discone Antenna Designer

Free Online Engineering Calculator to quickly estimate the geometric Size of the Parts of a Discone Antenna. International as well as for USA.

www.changpuak.ch

....google is your friend

 

[prcguy]

A Discone by design is a non resonant antenna that covers about 8 octaves of frequency range and the Sirio SD1300 appears to be the common size where the Discone part starts around 110MHz which would then cover up to about 880Mhz by design. They actually go much higher but the radiation pattern swings upward above the horizon about halfway through the basic frequency range. In other words your antenna is already optimized for the 108-180Mhz range.

The top whip on some models resonates around 50MHz and some at 27MHz, not sure about the Sirio but you can remove the whip if you don't need to go lower than 100MHz.

 

[kamilkamien]

Thank you for this idea. My friend advised me to do the same, so I removed the top radiator. In fact, it has never even been installed, so I don't know how it behaves with this radiator.
The idea of fitting my Discone is very simple. As a test, I hung a self-made GP antenna on my mast, which I made for a frequency of 140 MHz. I connected it to a scanner and certain frequencies that I had previously listened to on Sirio are much clearer in reception on my straight GP.

I checked the link to the calculator. I would have to remove about 17 cm from the counterweights and 4 cm from the arms of the flat radiator. I wanted to make sure whether such a procedure makes sense because it will be difficult for me to return its dimensions to their original state.

 

[prcguy]

Thank you for this idea. My friend advised me to do the same, so I removed the top radiator. In fact, it has never even been installed, so I don't know how it behaves with this radiator.
The idea of fitting my Discone is very simple. As a test, I hung a self-made GP antenna on my mast, which I made for a frequency of 140 MHz. I connected it to a scanner and certain frequencies that I had previously listened to on Sirio are much clearer in reception on my straight GP.

I checked the link to the calculator. I would have to remove about 17 cm from the counterweights and 4 cm from the arms of the flat radiator. I wanted to make sure whether such a procedure makes sense because it will be difficult for me to return its dimensions to their original state.

There should not be a lot of difference between a Discone and a 1/4 wave ground plane within the Discones first few octaves of design range. I would expect a ground plane to be maybe 1-2dB better at best and that will vary slightly over the frequency range.

 

[kamilkamien]

There should not be a lot of difference between a Discone and a 1/4 wave ground plane within the Discones first few octaves of design range. I would expect a ground plane to be maybe 1-2dB better at best and that will vary slightly over the frequency range.

So you're thinking about leaving it as it is and not doing anything with the discone?

 

[prcguy]

So you're thinking about leaving it as it is and not doing anything with the discone?

I don't think you can improve a Sirio SD1300 in the frequency range you mentioned and shortening elements would make it worse.

 

[bbo14]

I assume that by modifying you are attempting to get more gain at your desired operating range. Forget it. A discone is a broadband antenna. They have, as prcguy pointed out, a wide range of frequencies. I remember using them in the Army. Ten octave range was posted. prcguy may be more spot on with eight octave. You are not going to get any more "gain" out of them by attempting to narrow them up and you may be "shooting yourself in the foot" if you don't know how to modify one. The formula for a discone is: top horizontal part, 70 percent of a quarter wave diameter, bottom skirts, quarter wave from top to bottom, insulator, no more than 1/10th wavelength. These are based on the lowest frequency desired. So, .7 x 1/4 wave across the top for the top elements and 1/4 wave for each lower rod. BTW: if you want to have some fun, mount it on an insulated mast, turn it sideways (makes it horizontally polarized), and turn it to see if signals null. Just for fun of course. The signals you are interested in are vertically polarized.

 

[merlin]

All discone antennas are very broadband and need no tuning. If you are looking to restrict the bandwidth you get, A VHF bandpass filter would be my recomendation.
One thing I have seen done is removing the top (horizontal) section and replaceing with a 1/4 wave verical rod.
That in effect will turn the antenna into a ground plane.

 

[Ubbe]

Those normal sized discones will have a low SWR over a 1:10 range, 100-1000MHz, but the antennas direction pattern changes and the antenna works best in a 1:5 range, 100-500MHz. Going higher will reduce the antennas efficiency and will have a lower gain to the horizon. The lower radials are set to the lowest frequency and the upper horizontal elements are set to a frequency in the middle of the frequency range.

If you want to improve VHF-AIR you can try and increase the horizontal elements in the direction to the transmit source. If you have a constant signal from an ATIS channel you can easily check if you get any improvement by lengthen the horizontal element or two that points in that direction. You can use a screw block that are used to connect two wires together to extend the element to twice its length.

If everything you receive are from one general direction then you can remove the horizontal elements that points in the opposite direction in a half circle as those resonate and take away some of the signal.

/Ubbe

 

[kamilkamien]

Those normal sized discones will have a low SWR over a 1:10 range, 100-1000MHz, but the antennas direction pattern changes and the antenna works best in a 1:5 range, 100-500MHz. Going higher will reduce the antennas efficiency and will have a lower gain to the horizon. The lower radials are set to the lowest frequency and the upper horizontal elements are set to a frequency in the middle of the frequency range.

If you want to improve VHF-AIR you can try and increase the horizontal elements in the direction to the transmit source. If you have a constant signal from an ATIS channel you can easily check if you get any improvement by lengthen the horizontal element or two that points in that direction. You can use a screw block that are used to connect two wires together to extend the element to twice its length.

If everything you receive are from one general direction then you can remove the horizontal elements that points in the opposite direction in a half circle as those resonate and take away some of the signal.

/Ubbe

In my location, I have an airport from which ATIS unfortunately does not reach me, but I have a distant base station that conducts continuous conversations. I will perform the procedure you mentioned and extend one arm by its entire length. If this treatment brings a positive result, can I do the same with other courses? I have one airport in the east and the other in the north, and it would be good to hear them both in a larger range. To what extent does such a modification change the directionality of the antenna? Will north and east work? Will North and South work or will the best effect be achieved by directing the modification only in one direction?

 

[Ubbe]

To what extent does such a modification change the directionality of the antenna? Will north and east work? Will North and South work or will the best effect be achieved by directing the modification only in one direction?

A discone only receives by one or two elements in the direction of the signal. It would be beneficial to remove all other horizontal elements, but if you receive airborn aircrafts then those will probably be in many different directions. If you modify one element then that will affect the signal in that specific direction where it points. Normally you have one horizontal element that receives a signal and 7 other "dead" elements that sits in parallel that radiates a part of the signal that then cannot go into the coax.

The biggest gain would be to have more frequency tuned horizontal elements in the directions that matter the most and those untuned original elements where you receive higher frequencies and any untuned element will then not suck out as much of the signal. Best would be to just unscrew elements in the directions where you will not monitor any signals.

The lower skirt elements are only a reflector and needs to be long enough for the lowest frequency. If you double the length of a horizontal element it will work worse at higher frequencies above 300MHz but better at its lowest frequency range.

For testing I would first check the signal strength from one transmitter with a steady signal strength, then unscrew all horizontal elements but the one pointing at the transmitter and compare signal strength. Then extend that element and compare again. Then install all other elements again and compare. Then you will have a result that will indicate if it will be worth doing any modifications and which elements to modify.

/Ubbe