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Telescopic Antenna's

PostPosted: Tue Jan 03, 2017 3:27 pm
by DARREN4963
Telescopic Antenna's

Some people have a misconception that a telescopic antenna needs to be fully extended to receive the best reception. This is not so.

Basically every frequency has a resonant point. However there is no need to have a antenna for each point just as long as the antenna is the right length for the band. Example being the centre of the civil air band is 127.000mhz so using the formula 234 divided by 127 (the optimum freq) you get 1.8 feet. Times that by 12 and it gives you 22.1 inch's. There for a vertical antenna for civil air band needs to be 22 inch's roughly.

So if you are using a telescopic antenna and you want to it optimised for Civil air it needs to be around 22 inchs.

Like wise if you wanted to use the same antenna to now listen to the Mil air band at 22 inch's it wouldn't be as efficient and so that is why they are expandable ! so you can make it shorter so going on the same principle middle of the Mil air band is say 325mhz (225 to 400mhz is the band) so 234 divided by 325 gives you 0.72 feet now times that by 12 and you get 8.64 inch's so to listen to the Mil band with the same ant you need to reduce it to around 9 inch's.

So not trying to teach anyone to suck eggs but if you are using a telescopic (which work great by the way) use the formula to ensure the antenna is optimised for the freq your listening to.
Sadly the manufactures fail to explain this as it would mean them putting a scale on the antenna i guess and that costs money.

Re: Telescopic Antenna's

PostPosted: Tue Jan 03, 2017 4:17 pm
by m0lsx
I have a rather old middle loaded telescopic antenna designed for scanner use.
I have long lost the instructions, but it was designed to be used base loaded at some frequencies..Top loaded at others & mid loaded at others. And it's a damn fine antenna, despite only being about 2 foot long when fully extended.

Re: Telescopic Antenna's

PostPosted: Tue Jan 03, 2017 5:29 pm
by DARREN4963
Using that formula you should be able to work out what length it needs to be extended or retracted to for best reception on the freq or band your scanning.

Obviously if your scanning various bands ie civ air and then moving up to pmr etc it wont work as well

Re: Telescopic Antenna's

PostPosted: Tue Jan 03, 2017 6:21 pm
by G4RMT
Darren - you need to read a little around the forum before jumping in - Alan does rather know that. We've had numerous topics on the features and benefits if telescopic antennas and in this example - unless you have an analyser, then there isn't really a formula you can use is there? Middle and top loading requires more specs or measurements.

Re: Telescopic Antenna's

PostPosted: Tue Jan 03, 2017 6:36 pm
by milly
...and on the subject of putting the lengths has been done by several manufacturers over the years. Revex and Maldol are two that I recall.

Unusually, the Maldol one had two flexible joints which was also very useful.

Re: Telescopic Antenna's

PostPosted: Tue Jan 03, 2017 6:51 pm
by DARREN4963

Re: Telescopic Antenna's

PostPosted: Tue Jan 03, 2017 7:17 pm
by Staffs Paul
DARREN4963 wrote:In response to your reply regards Alan ?

Are you implying possibly Alan is taking the piss ?

I have been a member of this group for 1 day and so far it would appear its not very friendly

This forum is very friendly and helpful and Alan I have found to be one of the most helpful people online on radio forums. i cannot see anywhere anything that he said that could be construed as taking the piss he didnt even direct anything at you he just posted his own experience on the topic you posted. The QRZ comment from yourself though is wrong IMO. People have only been replying and giving input to you on the threads you have started.

Re: Telescopic Antenna's

PostPosted: Wed Jan 04, 2017 12:16 pm
by m0lsx
No, I was not taking the piss. I was simply saying I have an antenna designed long ago when scanners were simpler & much more basic than now.
It, as I remember it. Is designed to be used top loaded on low VHF, base loaded on UHF & mid loaded on bands like Air, marine etc.
It makes a poor modern scanner antenna, unless you want to listen to a single band. But it is a good antenna, when set up correctly.
I think what G4RMT was referring to was the fact I know the basics of antenna design.
I started building antennas back in the 70's when I was broadcasting.
I am sure you know that better than me what a loading is & does, especially since you are designing antennas. But I am not sure why you feel that your formulae would help me on an antenna that could have that coil at it's base, middle or at it's top.
It was my understanding that the loading coil would create some phase shift in the antenna & cancel at least some capacitive antenna reactance.

Re: Telescopic Antenna's

PostPosted: Sun Jan 15, 2017 10:12 pm
by LandingGearLaurie
My telescopic antenna is poor in comparison to my Garex Flexi-Whip made by Spectrum Communications. Its Airband specific and it is a real handy bit of kit. It pulls in from all over the place and in imho the best mobile antenna to have.

Fainling that, a Spectrum Communications SlimG will out do anything. I get 2 way comms from Gatwick and Heathrow towers and they are 16 and 26 miles away respectively.

I still have a telescopic antenna and will use this for low end CB stuff but the two mentioned above sort my Air and Marine out nicely!

Hope this helps


Re: Telescopic Antenna's

PostPosted: Sun Feb 12, 2017 9:06 pm
by M3SWL
Thanks for that formula Darren. I use 3 different lengths telescopic antennas. I can now try to mark them.