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CB mag on PMR?
Topic Started: Wednesday, 28. May 2014, 08:55 (733 Views)
Mudslinger
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This might sound like a daft question, but is there any reason why I cannot use a CB magmount base on PMR 446?

Obviously I would need a different whip and connector on the end of the coax to get it to work.

Has anyone tried it?
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BigTone
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26 Charlie Tango 4498
The only one is most CB mag mounts have a 3/8 fitting which restricts you to a 1/4 wave (About 6")
However if you have a magmount with a SO239 fitting (As used by the Sirio HP4000 et al) then any 70 cms Ham antennae can be usable.
Of course that is illegal
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S4b0t3ur
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26 Charlie Tango 985

Hi,

Or you could use one of these (see link)

http://www.ebay.co.uk/itm/MR700-2-70-38th-THREAD-DUAL-BAND-VHF-UHF-MOBILE-ANTENNA-/130971893056?pt=UK_Mobile_Phones_Communication_Radio_Antennas&hash=item1e7e887d40

regards
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Offplanet
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26 Charlie Tango 104

One problem I can see is the fact that RG58 is grossly lossy at UHF freqs - especially considering the very poor quality RG58 I've seen used on many of the cheaper mag bases.
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Deleted User
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Eres what i made up works well.....................

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Mudslinger
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Thanks for the help everyone. Obviously this is going to be for listening only. ;)

I was thinking of putting better coax on, however having looked at some loss tables RG-58 has less loss at UHF than the RG174 than some of the ready made magmounts are supplied with.

What sort of coax should I really be using? If it works well at CB as well then that would be a bonus.
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DJS169
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163 Charlie Tango 060

ofcom have removed the restriction of a fixed antenna so you can now use an outdoor antenna. check out http://en.wikipedia.org/wiki/PMR446 (Radios may now have removable antennae as long as the ERP does not exceed 500mW)
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BigTone
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26 Charlie Tango 4498
If it works reasonable at 446 then 27 is no problem at all.
You are correct about RG174 at UHF its marginally better than wet string.
There are various types of RG58 so you need to check the specs. I believe double screened is the better. Mini 8 might be an option
What you need to balance out is losses v ability to go through the door without letting water in.
Another consideration is the connector, PL259s are not the best at UHF so it might be an idea to fit a 50 ohm BNC connector and use an adaptor to fit on the CB.
Also consider shortening the coax to the minimum you can get away with
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Radio Pixie
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26 Charlie Tango 052

If you are going to use a SO239 magmount be careful of the quality/make of it.

I have compared a Sirio mag to a Moonraker and a CTE Intl with a dummy load, and only the Sirio could manage a low SWR up to 450 MHz. The other two were useless.

As for coax, 3 to 4 metres of high quality RG58 is perfectly OK. It's only about 1.5 dB loss. Not the end of the world.

It's often quoted that PL/SO "UHF" connectors are lossy at UHF. But as long as you avoid adapters, use good quality plugs, and adjust the antenna to counteract the potential mismatch, you can get away with them Click for LINK

And that leads to using a 3/8" type magmount. If you are making an antenna for it, ie 1/4w or 5/8w, you can trim the antenna specifically to that mount, and this will counteract the length of the thread and the length of bared back coax inside the mount.

Edited by Radio Pixie, Wednesday, 28. May 2014, 18:12.
73 from Dave the Pixie 26CT052 - G7OPC - CB & Ham Radioaficionado :)

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Mudslinger
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Thanks Dave and Tony.

Some food for thought.

I will have to do some experimenting. Is a straight 5/8 wave going o be a good impedance match do you think or will it need some form of matching like a 5/8 wave at 27MHZ does?

Edited by Mudslinger, Wednesday, 28. May 2014, 19:42.
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BigTone
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26 Charlie Tango 4498
Simon, a 5/8 is actually a base loaded 3/4 wave.
The reason is 5/8 wave (or .64 wave) gives a low angle of radiation but doesn't match to coax well.
A 3/4 wave matches well to coax but has a high angle of radiation.
The coil is there to bring the impedance down to ~50 ohms

Just been rooting, here's a whip to make yourself. just change the fitting to suit your magmount

http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=22026
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Radio Pixie
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26 Charlie Tango 052

As Tony says a 5/8w does need to be matched. I have build a similar antenna to Tony's link and it does work. I built it first with solid copper wire, then got my father to build a version from a stainless steel whip. The antenna then was attached to a 3/8" thread and into a CB magmount.

Also experimented with a 5/8w with 4 turns at the base (coil being 10mm dia or less). One way to build this is to use a PL259 for 10mm dia cable and a 10mm dia PVC tube which conveniently slots into the plug. The coil can be soldered inside the plug or wound on the PVC tube just above the plug. Add 40cm length of wire. Adjust for best SWR, glue and seal, job done.
Edited by Radio Pixie, Thursday, 29. May 2014, 08:41.
73 from Dave the Pixie 26CT052 - G7OPC - CB & Ham Radioaficionado :)

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