a****f Dipole question

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Banus_radio
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a****f Dipole question

Post by Banus_radio » Tue Jan 07, 2020 11:14 pm

On aareffs single dipole page if you scroll down to the ‘simple and easy’ text it says they install a coil to reduce a dipoles natural impedance of 73 ohms down to 50ohms.

I didn’t think this was possible without a matching transformer line or network, how can a single coil reduce impedance on an antenna without affecting other parameters. Does anybody know the details of this coil ??

https://www.a****f.com/en/tuned-dipole- ... r-antenna/

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Re: a****f Dipole question

Post by Banus_radio » Tue Jan 07, 2020 11:15 pm

https://www.a a re f f.com/en/tuned-dipole-fm-transmitter-antenna/

Remove the spaces :-)

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Re: a****f Dipole question

Post by Albert H » Tue Jan 07, 2020 11:38 pm

The answer is that you can't. The correct way to match a single dipole is with either a gamma match or a Pawsey stub. The stub method allows the use of just the coax, and will convert the unbalanced feeder to balanced for the antenna, and can also do the impedance transformation.

That said, you could use an "autotransformer" configuration to change the antenna impedance, but it wouldn't resolve the unbalanced to balanced issue.

If you have any confusion, just take a look at the ARRL Antenna Handbook. It gives all the calculations and tweaks to get aerials working properly. Paul Hollings obviously hasn't bothered to read the book, or he wouldn't make silly claims about a "matching coil".

Anyway - why on earth would you want to use a useless aerial like a dipole? A simple J-pole will outperform a dipole easily. Other options include verticals with groundplanes - they can look like a CB aerial from the ground, so you're less likely to "lose" the gear.
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Re: a****f Dipole question

Post by Banus_radio » Wed Jan 08, 2020 12:53 am

Yeah I do like a Pawsey stub and have favoured them over other methods back in the day !!
I wasn’t aware I’d did the impedance transformation though ? I just thought it was a balun only, eg not a transformer ???

Also how on Earth is mr hollings making two sets of elements cover the entire band with a swr below 1.5 ?

I remember a dipole only having about 2.5mhz bandwidth below 1.5, I’m intrigued how he does it.

I love how he says it’s truly unbalanced by added a ferrite core lol.

And lastly look at his 10 bay dipole setup lol. Wilkinson coax combiners all the way through ! Shocking !! Claims it can take a 1.6 KW yet the first feeder cable is rg213.
Am I right in saying rg213 can only handle around 700-800w at 100mhz ?

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Re: a****f Dipole question

Post by nrgkits.nz » Wed Jan 08, 2020 3:45 am

That photo of Hollings dipole gives the impression that its some cheap FM receive antenna that the average Joe Blogs would use to improve his reception at home, doesn't give the appearance of a professional broadcast dipole at all.

The "coil" he installs will get very hot at 200W, even more so if there's a rotten VSWR or matching problem, possibly melting into its plastic housing/insulation.

Reading some of the stuff on there really does show his in-competence.

RG213 gets very warm at 500W and at 1.6kW its going to get very hot. The dietetic (which is not teflon) will eventually start to melt depending on the heat and reflected power, causing the coax to no longer be 50ohm, the VSWR will go up which will cause even more heat - a run away situation until the PA folds back.

For that sort of power, you need LMR600 at a minimum, preferably Heliax with 7/16 DIN connectors/sockets to deal with the power, not the PL259 and N types Hollings uses

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Re: a****f Dipole question

Post by Banus_radio » Wed Jan 08, 2020 5:25 pm

So today I download antenna modelling package eznec demo and for the life of me cannot get a dipole to be broadband.
Folded dipoles are showing around 270 ohm and even with a 4:1 balun it’s nowwhere near 50ohm, sits as swr1.5 with about 4mhz bandwidth

So I decided to melt down my old skywave folded dipole antenna to see what was inside as it boasts a 20mhz bandwidth below swr 1.5. Now although it’s not for broadcast band it should give me an idea of what’s but going on. This one was for 170mhz though.

Inside was a coil of wire folded back on itself and wrapped around the sheath of the coax

I’m not 100% sure but it looks like one end of the the coiled wire went to the centre core of coax which connected to one element and the other end of the coil went to the bottom element. The screening of coax only connect to the bolt which shorts to the centre zero point of the the folded dipole via the boom arm

Any ideas what this match is ? And any diagrams for one ?
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Last edited by Banus_radio on Wed Jan 08, 2020 5:31 pm, edited 1 time in total.

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Re: a****f Dipole question

Post by Banus_radio » Wed Jan 08, 2020 5:26 pm

The loop of wire is very thin like single core BT telephone wire

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Re: a****f Dipole question

Post by Banus_radio » Wed Jan 08, 2020 8:52 pm

C0BD7395-7F05-482E-80F6-204AFAA1A08D.jpeg
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Re: a****f Dipole question

Post by Albert H » Sat Jan 11, 2020 1:44 am

That looks horrible!

I use Andrews Heliax FSJ1-50A for anything over 200 Watts. It's nicely flexible and easy to install and terminate. When I get to 5 kW, I start to use HL4RPV-50A, which remains cool at up to the biggest installation I commonly do at 25 kW.

The SO239 / PL529 is useless at VHF frequencies. For the clandestine installs (some folks call them "pirates"), I normally use RG58 up to about 25 Watts and RG213 above that. I use 50Ω BNC connectors for the RG58, and "N"-types for RG213. Really low power stuff and link receivers use SMA connectors or "F"-types.

Vertical aerials are always groundplane types. I simply can't be bothered with flaming dipoles - they're OK for the kids, but if you want to do it properly, look at aerials with real gain. Also you'll stop having gear removed from blocks when you start using effective groundplane aerials, because the field strength in the block will be low (reducing the risk of overload or mixing interference to radios in the block), and the cretins trying to find and remove your gear will be looking for dipoles! You'll get much more RF out with a proper vertical than you will with a dipole - you can get much more signal for the same transmitter power!
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Re: a****f Dipole question

Post by Banus_radio » Sat Jan 11, 2020 6:54 am

Albert do you know what the coil of wire is doing in the pic above ?

It’s folded back on itself so I don’t think it’s an inductor as it’s cancelling itself out I think ?

The fact that the bottom folded element is not shorted the ground makes me wonder if this is not actually a folded dipole but actually a unipole but with a ground counterpose hanging off the boom arm?

On meter the antenna has 20mhz bandwidth, if I remove the coil it’s has no low dips on analyser and is useless.

Any ideas ?

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Re: a****f Dipole question

Post by Bton-FM » Sat Jan 11, 2020 10:42 am

Albert why don't you use RG58 +25 watts?

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Re: a****f Dipole question

Post by Banus_radio » Sat Jan 11, 2020 11:07 am

Bton-FM wrote:
Sat Jan 11, 2020 10:42 am
Albert why don't you use RG58 +25 watts?
It’s very lossy. I think around 30m of rg58 can half your power.

And at higher powers say 50watts up it gets nice and warm.

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Re: a****f Dipole question

Post by Bton-FM » Sat Jan 11, 2020 11:58 am

Ahhh ok .I knew was lossy but not about it getting warm.How many meters of coax would you say an instillation requires?

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Re: a****f Dipole question

Post by Albert H » Sun Jan 12, 2020 1:26 am

How long is your piece of string?

I've used as little as 3m of coax or as much as 80m (going up a tower).
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Re: a****f Dipole question

Post by yellowbeard » Sun Jan 12, 2020 6:36 am

That coil is his 4:1 balun, it's like a 1/4 wave stub and it's similar in principle to what Harry Lythall does here with the coils on the wooden formers:
http://85.226.178.87/antennas/7db-colinear.htm
I would bet the gauge of that wire limits the power handling of the antenna.

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Re: a****f Dipole question

Post by Banus_radio » Sun Jan 12, 2020 5:15 pm

yellowbeard wrote:
Sun Jan 12, 2020 6:36 am
That coil is his 4:1 balun, it's like a 1/4 wave stub and it's similar in principle to what Harry Lythall does here with the coils on the wooden formers:
http://85.226.178.87/antennas/7db-colinear.htm
I would bet the gauge of that wire limits the power handling of the antenna.
Do you have any more info on this ?

It’s not the typical 1/2 wavelength of coax folded back on itself to form a typical 4:1 balun.

This is single core wire.

Do you have any info on what length this folded bit of wire should be ?

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Re: a****f Dipole question

Post by yellowbeard » Tue Jan 14, 2020 5:08 pm

Most likely its a half wave long folded back on itself to make a quarter wave stub just like your drawing. It's the same as the half wave coax just cheaper and nastier.

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Re: a****f Dipole question

Post by Electronically » Thu Jan 30, 2020 4:22 pm

Paul hollings used to sell dipoles with a 50 ohm dummy load resistors soldered inside pl259 lol. My mate only noticed after he took his dipole down to a different location . the pl259 socket came loose so he went to unsolder it. there it was 3 resistors in parallel lol

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Re: RE: Re: a****f Dipole question

Post by Electronically » Thu Jan 30, 2020 4:24 pm

Electronically wrote:Paul hollings used to sell dipoles with a 50 ohm dummy load resistors soldered inside pl259 lol. My mate only noticed after he took his dipole down to a different location . the pl259 socket came loose so he went to unsolder it. there it was 3 resistors in parallel lol

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That was probably to make my mate think he's got a brilliant swr across the whole band lol

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