Dipole choke ..

[FMEnjoyer]

Found this for 2m band to keep CMC off coax / feeder.

https://q82.uk/buildadipole

Not quite sure how many turns would be for another band but I presume one less if dropping in frequency a bit.

[EFR]

I say 6 turns.

[yellowbeard]

If you're going to do that you want more turns not less - and how many depends on your frequency. A Pawsey stub would be a better solution, it also is tuned but there would be less pricking around with the diameter of your boom and the diameter of your coax, one calculation and you're done:
http://www.gareth.net.nz/nrgworkshop/ha ... aerial.htm

[King Croccy]

Would it still work with a metal boom arm?

I can't see it being very good practice to use pvc pipe for the boom. It will just flop around in high winds

I've built a pawsey stub balun before and that worked well with a dipole. It fitted straight into the metal boom and was nice and weatherproof

Also (for some reason) the elements needed to be cut quite a lot shorter than you usually find without a balun. It almost looked like a 2m band antenna when I had finished cutting (for lowest swr).

[Albert H]

Found this for 2m band to keep CMC off coax / feeder.

https://q82.uk/buildadipole

Not quite sure how many turns would be for another band but I presume one less if dropping in frequency a bit.

No!!! MORE turns as you go down in frequency. Either 6 or 7 turns would work. The boom arm needs to be longer, too at Band II.

Bear in mind that a VHF dipole is the aerial most used by pirates, and is a dead giveaway to the location of your rig!

It's MUCH better to use a longer vertical (with groundplanes) and a base loading coil. Matched correctly, a ⅜, ⅝, or ⅞ vertical will give useful gain over a dipole, and a better radiation angle. It will also look like a CB aerial to the numpties trying to find your gear.

I used to use a ⅝th-wave vertical with groundplanes, and the signal inside and immediately around the block was pretty weak - you wouldn't think the signal was coming from that block! A kilometer or so away, the signal was huge!

[Wk-end]

Found this for 2m band to keep CMC off coax / feeder.

https://q82.uk/buildadipole

Not quite sure how many turns would be for another band but I presume one less if dropping in frequency a bit.

No!!! MORE turns as you go down in frequency. Either 6 or 7 turns would work. The boom arm needs to be longer, too at Band II.

Bear in mind that a VHF dipole is the aerial most used by pirates, and is a dead giveaway to the location of your rig!

It's MUCH better to use a longer vertical (with groundplanes) and a base loading coil. Matched correctly, a ⅜, ⅝, or ⅞ vertical will give useful gain over a dipole, and a better radiation angle. It will also look like a CB aerial to the numpties trying to find your gear.

I used to use a ⅝th-wave vertical with groundplanes, and the signal inside and immediately around the block was pretty weak - you wouldn't think the signal was coming from that block! A kilometer or so away, the signal was huge!

This is word! 100%

[FMEnjoyer]

If you're going to do that you want more turns not less - and how many depends on your frequency. A Pawsey stub would be a better solution, it also is tuned but there would be less pricking around with the diameter of your boom and the diameter of your coax, one calculation and you're done:
http://www.gareth.net.nz/nrgworkshop/ha ... aerial.htm

very nice info thanks kindly, does not seem to difficult. I also heard adding bead help. I don't really do any radio these days but like all types of it, always liked GPA, I always had better luck with quick matching without any baluns, but they can also suffer cmc on coax even though supposedly less likely.

I also read about adding 15-20 ferrite beads of correct size over coax at antenna end is good for about 2K ohm.

Piece of coax is cheap and effective.

[FMEnjoyer]

Found this for 2m band to keep CMC off coax / feeder.

https://q82.uk/buildadipole

Not quite sure how many turns would be for another band but I presume one less if dropping in frequency a bit.

No!!! MORE turns as you go down in frequency. Either 6 or 7 turns would work. The boom arm needs to be longer, too at Band II.

Bear in mind that a VHF dipole is the aerial most used by pirates, and is a dead giveaway to the location of your rig!

It's MUCH better to use a longer vertical (with groundplanes) and a base loading coil. Matched correctly, a ⅜, ⅝, or ⅞ vertical will give useful gain over a dipole, and a better radiation angle. It will also look like a CB aerial to the numpties trying to find your gear.

I used to use a ⅝th-wave vertical with groundplanes, and the signal inside and immediately around the block was pretty weak - you wouldn't think the signal was coming from that block! A kilometer or so away, the signal was huge!

Thanks for correction, never heard of 3/8 antenna though.

[Albert H]

The ⅜-wave aerial is OK, but has less gain than its bigger relatives.

There's also the 0.66-wave vertical that can (just about) work without groundplanes.

Practically, the best way is to experiment in your backyard, with a low power transmitter that accurately matches 50Ω and a good SWR bridge. My old test rig used a tall stepladder with a pole strapped to it in the middle of my garden. I had enough room so that the antenna under test would be about four wavelengths away from the nearest building. I could test various antenna designs and get them to match accurately before they were taken "up the block" and have lots of power fed to them. This allowed me to experiment with various matching schemes and ways of feeding energy to the radiator(s) - the "gamma match" became a favourite for a while - and I developed several useful aerials, each with their own characteristics. Experimentation really is the best way to learn!

[zulu53]

I wonder what type of approach do the FM antenna manufacturers use in order to get dipoles that are at the same time: broadband; can withstand high power (sometimes up to kW) and display very low VSWR.
Do they follow any of the approaches suggested in this thread?

[Albert H]

There aren't any truly "broadband" aerials that work well. As with all engineering, it's a compromise. You can widen the bandwidth of an aerial by using "fatter" elements, but the antenna gain is usually compromised too.

Most commercial aerials are built to frequency. The old "Comet" ⅝-wave aerial was tuned by extending and shortening the radiating element. Again, this was a compromise because the length of the groundplanes was optimised for the middle of the band, and the VSWR would worsen towards either end of the band.

Some manufacturers claim that their folded dipoles are "broadband", and one of the commercial models I tried had a reasonably flat SWR for about 3 MHz either side of the "natural" frequency. However, it was lossy, and didn't radiate as well as a simple "cut to length" dipole with a Pawsey Stub.

Whenever I've done installations, the antenna is ALWAYS tuned to frequency - the cheapest RF amplifier is an effective aerial!

[yellowbeard]

Thicker radiators taken to the extreme:

http://admin.mb21.co.uk/tx/userimages/2 ... 104150.jpg

Those 45 degree sections are 1/4 wave long, say 75cm and look to be 10 or 15 cm thick - this is the lengths (or thickness) the BBC goes to to cover 12MHz bandwidth at Tacolneston:

http://tx.mb21.co.uk/gallery/gallerypage.php?txid=724

I would imagine the power handling on that sort of antenna is down to the feeder and connectors they use not the metalwork and there are 32 of them guys on that mast, that'd take a lot of melting with RF...

[FMEnjoyer]

Nice post yellowbeard, galvanised steel ? the worst metal for an antenna. Copper would go green surprised they are not aluminium.

[reverend]

If you're going to do that you want more turns not less - and how many depends on your frequency. A Pawsey stub would be a better solution, it also is tuned but there would be less pricking around with the diameter of your boom and the diameter of your coax, one calculation and you're done:
http://www.gareth.net.nz/nrgworkshop/ha ... aerial.htm

+1 for the Pawsey Stub. They're cheap, easy to build and work well. They also have a secondary benefit of reducing second harmonic radiation as the stub becomes a short-circuit at even multiples of the design frequency.

[radionortheast]

Easier to make fewer turns with a larger diameter you could damage the coax, you can get bandwidth with a dipole with 10mm tubing or slightly less, i’m not sure a choke alone does much for the bandwidth, with a match it works, also works well in a reciever. The raidation pattern will mostly be the same, I only have very low power here, same db level was recieved on another reciever from 88-100, values of capacitance had to be changed in the filter between 93-95.

You can get aerials like, folded dipoles, make an open jay, something which is made from a half wave tube, then quarterwave runing along side of by afew cms, (this can be some wire, a folded dipole can be bent out to get the lenght of the main tube), you can make something that sees low swr across the band, the radiation pattern will differ alot.
I did make one worked well at a specific frequency, better than anything else, then static built up during a lighting storm blew the device, have to make sure it is dc short, reduces bandwidth alot. End fed aerials the coiling of the choke becomes more important part of the coax becomes the aerial, their good if you can get an aerial outside, they can be exstreamly light weight.
The aerial here is mostly used for recieve with very low power, the actual choke is 2 turns 9cms total, the match is an insulated wire about 20cms long, comes from the feedpoint of the top element then runs down along the bottom element before connecting to the feedpoint of the other element. 75ohms cable is used theres no reason why 50oms can’t be used, 75ohm cables have a soild inner conductor, get a good connection through the f connector without any screws, then its easy to connect to the sma with a converter to the tef668. I don’t know if a pawsey stub would cause a loss in a reciever, I will have a look at that page, they do work very well. I will problalby have a go at making the end fed half wave when I get the materials, which will probably be another folded dipole.