Modifying a dual band rooftop antenna

[mainetrunk]

Hi all.. Simple question, I hope..
I have 1 of those antennas that are made by every brand.. It sits on the post, 8 elements that stick out horizontally, and the long vhf elements that extend downward, at an angle...
Here's the fact - my uhf mag mount antenna is way more receptive to uhf than the dual band antenna.. Same height, 20feet apart..
Question - would cutting the horizontal elements shorter work better for the uhf?
If so, what length?
Thank you

 

[mmckenna]

Hi all.. Simple question, I hope..
I have 1 of those antennas that are made by every brand.. It sits on the post, 8 elements that stick out horizontally, and the long vhf elements that extend downward, at an angle...

Pictures would help, but that sure sounds like a discone. The elements are not setup the way you think...

Here's the fact - my uhf mag mount antenna is way more receptive to uhf than the dual band antenna.. Same height, 20feet apart..

Yes, that's very likely. Depending on what kind of UHF antenna you have, it may actually have some gain compared to the discone (discones generally are accepted to be 0db gain antennas), and the radiation pattern is probably better on the UHF antenna.

Question - would cutting the horizontal elements shorter work better for the uhf?

Unlikely. It's still going to be a discone (I think…) and it's likely not going to improve the performance to start messing with it.

If so, what length?
Thank you

Discones are not great performers. They work well for some people, but not for everyone. The beauty of them is that they are extremely broad banded. The lack of gain and funky radiation pattern as you go up in frequency usually result in mediocre performance.

It sounds like you are up against the limits of what a discone can do for you. You have a couple of options to boost performance:
- Increase height
- Use higher grade coax cable to get more signal to your radio.
- Use band specific antennas that have some gain.

 

[mainetrunk]

Discone, exactly... That's the 1st name I knew it as, then, other brands grabbed it too... I'm scanning 450-485 mgs for uhf.
Junk?

 

[mmckenna]

Discone, exactly... That's the 1st name I knew it as, then, other brands grabbed it too... I'm scanning 450-485 mgs for uhf.
Junk?

No, not junk, but sounds like it's not the right antenna for what you are doing.

Remember that a discone has 0dB of gain, so using a UHF antenna that does have gain should give you much better performance if all the other variables are the same (coax, location, height, etc).

Getting a UHF base antenna designed for the 460MHz range with a few dB of gain should help you. Something like this would give you 5dB of gain and get a lot more signal to your receiver:

Laird TE Connectivity FG4605 - GMRS Omni-Directional Base Antennas

460-470 MHz omnidirectional fiberglass antenna. Gain (dB) 5, Direct Termination N Female, Overall Length 6.33'

theantennafarm.com

 

[paulears]

Why do the military and civil air technical folk use discones, when they have 0dB gain? Because they have 0 at a very wide range of frequencies. Antennas with gain, which business and hams prefer, perform better on their narrow range of frequencies. Sometimes gain is NOT the requirement. One of my gainy antennas on the office lets me hear hams 10 miles away, despite being at sea level in the harbour. If I try it on marine band, i hear zero ships from the same locations. It has quite a bit of gain at 145MHz and performs terribly at 156! You cant have high gain, AND wide band.

 

[mainetrunk]

10-4.. Guess I'll throw the discone up on the roof when it's not 100+ on a roof top. Lol.
Thanks

 

[Ubbe]

Question - would cutting the horizontal elements shorter work better for the uhf?
If so, what length?

Yes. The normal size discone works from 100MHz up to 500MHz but its change of direction pattern will make it unsuitable for higher frequencies. If you don't need the VHF airband to have full sensitivity then cut all elements, top and bottom, down to half size and it will work from 200MHz to 1000MHz. You can can scale it freely to any size as long as you cut all elements down to the same percentage of its original length. You probably don't need it to go to 1000MHz, maybe 850MHz, there's no sharp edge at the frequency limits, then cut down to a 65% length.

/Ubbe

 

[prcguy]

Why do the military and civil air technical folk use discones, when they have 0dB gain? Because they have 0 at a very wide range of frequencies. Antennas with gain, which business and hams prefer, perform better on their narrow range of frequencies. Sometimes gain is NOT the requirement. One of my gainy antennas on the office lets me hear hams 10 miles away, despite being at sea level in the harbour. If I try it on marine band, i hear zero ships from the same locations. It has quite a bit of gain at 145MHz and performs terribly at 156! You cant have high gain, AND wide band.

 

[prcguy]

Comparing a Discone to a multiband gain type antenna that has say, 5dBi on UHF and 3dBi gain on VHF, the Discone will probably have more gain over more frequencies due to the narrow band attributes of gain antennas. If the gain antenna has 5dBi gain over the 450-470MHz range it will have less gain than a Discone over the 225-400MHz mil air band as an example. If the gain antenna has 3dBi gain over the 150-160MHz range it will probably have less gain than a Discone over the 118-136MHz VHF air band.

If all you want to hear is confined to the 150-160 or 450-470MHz range then the gain type might be a better choice, but for most scanner folks we want or need more frequency range and you have to give up something for that.