discones on uhf why do the uhf signals fade even local ones.

[satob1]

hi everyone, ive tried every discone there is to try over ther years .,the problem people have in australia depending on what freq is the discones are no good on uhf from say 400mhz to 900 etc as they fade in and out. the only way to fix this is a scanner preamp from gre or jim .the discones are superfine on vhf everywhere from 30mhz and up 70 to 87 is also excellent as well also aircraft band 108 to 136mhz and 220 to mhz to 398.00mhz am fm modes.i was wondering what these dual discones are like ive seen them on ebay usa us ,ebay uk as well as ebay european as well .also they have a basic set up discone in the usa from transel and other ebay stores in the usa but what are they like.the discone with the elements droopping up and the elements drooping down look good but i dont know what there like .l,can anyone tell me if theyve tried any of these dual scanner discones on any scanner if so what was the results ,thanks david nsw australia

 

[nanZor]

From what I've been reading, discones have a very broad swr-bandwidth, typically 2:1 at the lower cutoff frequency to the second harmonic, and a 3:1 swr beyond that. Depending on your feedline loss, this may be a factor at uhf and above if your discone's lowest design freq is around 54 mhz or so.

In addition, I'm concerned about the elevation angles anywhere from 3x to 5x the lower cutoff freq. I've read everything from discones starting out looking at the horizon, (or even starting out at +20 degrees!) but having progressively higher elevation look angles as you rise in frequency - perhaps why they are popular at airports.

Maybe someone with antenna analyzer software that can handle the discone complexity can help resolve this.

For full-disclosure, let me say that I have never used or owned a discone - time for me to try one for sure. My first thought however would be to make sure your feedline is of high-quality, or perhaps try a smaller discone designed to start out at 300 mhz perhaps.

You piqued my interest now for sure to see if the claims I've read are true or not...

 

[prcguy]

I use lots of Discones and love them. They do have an upward radiation pattern at the highest end of their design frequency but like every antenna they are stable and don't change gain or radiation pattern to blame on fading. If they sway on the mast that's a mechanical mounting problem and not the antenna.

Fading experienced on a fixed antenna listening to another fixed antenna is not from the direct signal between the antennas fading, it would be caused by signals bouncing off the ground or other large surface and arriving at your antenna from another path and out of phase which causes the fading.

I suspect the 800MHz signals you are trying to receive are coming from a distance and both the transmit antenna and your antenna are not terribly high. If you have lots of unobstructed flat ground between you and the transmit antenna like no trees or houses to break up the reflected ground wave the problem can be worse.

Not much you can do except get an antenna with more gain to get the signals in a range where the worst part of the fade is an acceptable level or experiment with two antennas spaced far apart vertically for diversity reception. That's a big science experiment.

I have a "Double Discone" antenna from "Nevada" which seem to be popular in the UK. Its not a Discone and there is no specific antenna design called a Double Discone, its a name drummed up by someones marketing department.

The Double Discone is a poor implementation of a well known antenna called a biconical or bicone. It doesn't have the proper angle on the elements or the required matching components to be a biconical so its simply a fat dipole that resonates around 145MHz and a wast of cheap Chinese stainless steel that rusts by the way. Save your money for something else, I think you will be going backwards if you purchase a "Double Discone".

As you mentioned the Discone is great for most everything except 800MHz, why not get a nice big dedicated 800 antenna and combine with the Discone using a diplexer? That should give you the best of everything and hopefully bring up your 800MHz signals to an acceptable level.
prcguy