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Extending the Antenna; PMR 446
Topic Started: Wednesday, 6. March 2013, 15:16 (1,304 Views)
Tea Boy
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Afternoon All.

I just wondered if the antenna on a standard, basic PMR446 set is subject to SWR of some sort.

Having taken one apart i found that the antenna is just simply a length of solid core wire, so my plan was to lengthen said wire and run it up my 6/10 metre pole.

A simple project to carry out but not sure if i would need to check SWR?
As i understand it all antennas need to be tuned??

Any advice is appreciated

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VoG
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Tea Boy
Wednesday, 6. March 2013, 15:16
As i understand it all antennas need to be tuned??
Yup :thumb:

ANY transmitter needs to have a resonant aerial, matched to the frequency, or it it likely to suffer damage.
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Miko
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Hi Jason, it is possible that your performance could improve with an increase in antenna size, tho not guaranteed. I think if you intend to experiment, and thats what it's all about, then first thing is to measure the existing antenna and make any substitute a multiple of that measurement. If youcan beg borrow or obtain by devious means a swr meter that covers the amateur 70cms band you would probably be able to fiddle around and get a reasonable match. You wont harm the radio by keying up briefly to check the match as it is probably fairly low power anyway. Might try a telescopic car radio element as this would be very easily adjustable. Personally, don't think you'd get a terrific improvement , but you never know, at least it would amuse you for a while trying.............Miock
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Tea Boy
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Yeah it is just an experiment as you say, always up for an experiment or two

Like with 27 megs the antenna lengths are usually based around the wavelength of the band ie 36 feet. (1/4 wave antenna is 9 feet like the tank whip)

I found out that the wavelength of the PMR446 is 67.2cm and i figured that multiples of that would be ok to use. :shrug:
This is not based on my extensive knowledge of antenna theory as i have none!! but just research i have done.

Just realizing that these cheap PMR's don't allow the connection of an SWR meter anyway so I guess there is no real way to check!

Considering that the pair of PMR's I got only cost me £6.00 i reckon for £3.00 its good for a punt!!

I will be adding a UV3r or 5r to the kit which will allow for proper use of base antennas so i think i will give it a go.

Will post the results, cheers :thumb:
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Offplanet
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26 Charlie Tango 104

The most popular course of action that people take is to remove the standard antenna and replace it with a RF connector like a BNC socket or with a short pigtail of coax with a BNC or perhaps a S0239 on the end.

Within the radio, the centre pin of the new RF connector is connected to the point where the original wire antenna was connected and the outer part of the connector connected to the negative terminal on the battery.

The socket will then allow the connection of an external antenna - a simple centre-fed dipole (160mm each side for 446) being a good starter. The SWR of the dipole should be there or thereabouts, which is good, as the 500mW output of these radios is probably not enough to drive a SWR meter anyway :)

Coax must be short and of good quality as UHF RF leaks away rapidly on transmit and receive - the benefits of having the external antenna can quickly be negated by coax and connector losses ;)
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Tea Boy
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Top Banana Gary!!!

That's why I like this forum, there is always someone with a good idea to share!!

That makes a lot of sense to have a connector of some sort rather than hard wiring solid copper core wire.

Thanks all
:thumb:
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26 CT 356 Mike
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26 Charlie Tango 356

THanks for this, this has answered a questions i was just about to ask. Have you had any joy?
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Sgt Bilko
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108 Charlie Tango 302

This any good.

http://www.oocities.org/euro446/ants.html



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Tea Boy
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Raven220
Monday, 11. March 2013, 02:20
THanks for this, this has answered a questions i was just about to ask. Have you had any joy?


Hi Mike, I haven't managed to find the time to try, hopefully this week will see some progress, so many other things on so.....

Sgt Bilko
Monday, 11. March 2013, 09:16


Thanks for the link Paul, I have been looking at this website, good source of info.

There are various things in the pipe-line for the PMR at the moment, will try to keep a record via the Youtube and post as i go.
Will post the links

Cheers Guys :thumb:

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Tea Boy
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OK, so after a couple of hours in the workshop and the compulsory 5 cups of tea later I have come up with this...

I reused the housing of the original antenna and heat shrank the coax into the radio, the outer braid is soldered to the negative battery terminal and the inner core is soldered to the original antenna terminal on the circuit board.

Its a simple center fed dipole with 160mm legs

I will give it a try as soon as I can to see how it fairs.

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Edited by Tea Boy, Monday, 11. March 2013, 22:14.
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VoG
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Nice job. :thumb:

But don't forget what Gary said,
  • "Coax must be short and of good quality as UHF RF leaks away rapidly on transmit and receive - the benefits of having the external antenna can quickly be negated by coax and connector losses"
That co-ax is thin and will be extremely lossey on UHF, and it looks like you have about ten feet (three metres) of it. If you're planning to hold the Dipole overhead on a short handle you could probably reduce that length by ½ or even ¾

Every inch you can shorten it will reduce the Loss, increase the TX Power at the Feedpoint and improve the RX.
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Tea Boy
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Hi Steve,

I have got some better stuff RG 213 but it wasn't until after I had finished i realised that I had used the wrong length.

I should have known someone on here would have noticed my error, thought I could sneak it in under the radar...... :whistle:
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VoG
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Tea Boy
Tuesday, 12. March 2013, 09:56
thought I could sneak it in under the radar
Not a chance! :D
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canuck7
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Its tricky tryin to put an external antenna connector on these tiny 446 rigs, usually cuz there is'nt a lot of room for the connector. i resorted to using a sattellite female socket one one radio. just be careful before you go attempting it..
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