discone antenna alteration idea.

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Dispatrick

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I have a discone antenna with a vertical element extending off the top of the unit. Would adding a longer element to the top of the discone antenna affect the performance of the antenna? good or bad that is.
 

jonwienke

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The vertical element extends the low end of the frequency range of the antenna. Lengthening it will lower the bottom of the range, but may introduce a gap between the frequencies covered by the actual discone and the frequencies covered by the vertical.
 

ChrisABQ

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I have been dealing with analog simulcast issues for awhile. I had bought a Centerfire discone with a vertical element tuned to the MURS frequencies of 151 mhz. The analog simulcast channels that I monitor were creating TERRIBLE distortion and these departments recently changed frequencies to the 151 band on their simulcast systems. Today, I had the idea to "remove" the vertical element and it has all but eliminated the distortion. The discone radiation zone was coupling with the radiation zone of the top element causing the nasty distortion. Comms are normal now.
 

jonwienke

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Not necessarily. If the vertical element is resonant in the same frequency range as the discone, it is a stupid design and getting rid of it is probably a good idea because the vertical and discone signals will be out of phase. But if the vertical is tuned to lower frequencies than the discone, then getting rid of it will reduce the usable frequency range of the antenna.
 

Ubbe

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Simulcast is a special case. You try to receive only one tower to not get interferience from the others on the same frequency. You either use directional antennas or attenuate signal until you only receive that one tower. Most people try to use different antennas even using paper clips to reduce signals.

If you have an external antenna the easiest way is to use an inline variable attenuator.

The top element on some discones makes the antenna work as a GP antenna at a low frequency and as a discone on higher frequencies usually 100MHz-800MHz. If the top element is cut to 50MHz it will probably also help at 150Mhz. At other frequencies not at the cut frequency and 3x it should perform as a high impedance element and not affect much with the low impedance discone.

/Ubbe
 

jim202

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I have been dealing with analog simulcast issues for awhile. I had bought a Centerfire discone with a vertical element tuned to the MURS frequencies of 151 mhz. The analog simulcast channels that I monitor were creating TERRIBLE distortion and these departments recently changed frequencies to the 151 band on their simulcast systems. Today, I had the idea to "remove" the vertical element and it has all but eliminated the distortion. The discone radiation zone was coupling with the radiation zone of the top element causing the nasty distortion. Comms are normal now.
In a simulcast system, when you start having the distortion problems, it is caused by being able to receive more that one tower. When you removed that top vertical element, you actually reduced the gain of the antenna and as such reduced the amount of signal your receiver was able to hear.

It sounds like your location was in an ideal spot where reducing the signal level was enough to resolve the simulcast distortion from multiple tower sites. This is not going to work for every one. It boils down to location, location, location and where the simulcast towers are located to where you live.

An example of this is where I live. The 700 simulcast tower is about a mile from my house. The scanner worked fine on the workbench with the small pull up whip. I installed a small gain antenna up in the attic and now I get distortion about 30% of the time from one or more of the other towers. So I just added in some attenuation using an in line pad.
 

ChrisABQ

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I don't want to hijack this thread, but would buying a discone antenna (designed without a top element), perform better than the centerfire?
 

nosoup4u

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I put a 3db gain whip from an 800Mhz antenna in place of the stock element and it really improved my 800Mhz reception.
 

prcguy

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A top whip on a Discone should have a base loading coil designed to load the whip for a low frequency outside the Discone range and also to decouple the whip within the normal range of the Discone. Most scanner Discones cover about 100 to 800MHz with the top whip resonating around 50MHz.

The Centerfire Discone looks to be cobbled together as cheaply as possible and there is no loading coil on the whip to decouple it in the normal VHF/UHF range. Its more of a ground plane than a Discone.

I would think any scanner type Discone would work better to some extent but it probably won't be a drastic improvement.
prcguy



I don't want to hijack this thread, but would buying a discone antenna (designed without a top element), perform better than the centerfire?
 

AlphaFive

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Discone alteration

Tram Discone w/ top element. I needed to put it in my attic crawlspace. Would not fit. I decided to simply leave out the top element. I was concerned that I might be losing signal by having the element out, and not removing the copper wire in the top coil. I simply unscrewed the top assembly, and unwound the copper wire from the assembly. My Tram works on an equal level with three other mounted discones in the attic.
As they say in the television commercials, "Your results may vary". I would add that taking apart a perfectly good antenna made me feel like I was kicking a puppy, not good, nottt good.
 
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