aerials/antennas for handhelds

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Dayglow
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aerials/antennas for handhelds

Post by Dayglow »

Hi folks, newbie on the block.

I?ve mucked about with radio for many years off and on CB & marine VHF mainly, but not to any great depth of understanding, enough to solve whatever need I had at the time.

I have a puzzler for you, which I?m sure your group can easily resolve for me.

I?ve been using 446 PMR whilst on family holidays for some time now, using a variety of cheap PMR?s. My best budget radio being an Alan 456R.
I recently bought a TTi Tx1446 based on good reviews and being in possession of a radiated power meter thought I?d see what the difference between budget an semi pro radios was given the strict and restrictive design criteria laid down by the authorities.

I set the meter up on a table, placed the TTi1446 a fixed distance away and whilst at arms length pressed the transmit button. The meter was then adjusted to read full scale deflection. Call it 10.
I then replaced the 1446 with the Alan 456, same distance and attitude. Press transmit and obtain a reading of just 2 !
Now this seems strange to me, as I would have thought it should be close to the same reading as the 1446.
Un-deterred, I took apart the Alan and replaced the fixed antenna, which I suspected to be a coiled ? wave, with a BNC connector and a proper ? wave 70cm band aerial.
This obtained a reading of 2.5. A wide range Scanner aerial produced exactly the same reading.

So what?s going on? I then filed down the end of an old bicycle wheel spoke to fit into the BNC connector and trimmed it to 162mm. This time the meter reads 8! The same spoke fitted to the Tti1446 (has an SMA connector) produced 10, same as its own aerial.

So; smart, scientifically manufactured and tested factory produced aerial ?.rubbish
Rusty old bicycle spoke ?.5 stars perfect.

Anybody wish to venture an explanation?
(I should say that because I've just busted my hip in a mountain bike accident, I'm unable to range test just now, so there will be a further post to this before too long).
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Guzzy
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RE: aerials/antennas for handhelds

Post by Guzzy »

The built in antennas on the cheap and cheerful PMR446 radios are pretty pathetic really. The manufacturers are only allowed to ship them with 500mW ERP. It seems a lot of manufacturers simply use an inferior antenna design (that happens to be small and stubby - which people prefer apparently...) to achieve this 0.5W ERP. A lot of people find the range of these radios is significantly improved if they attach their own antenna.

Having said that, I'm sure not all of them are capable of putting out 500mW even with a good antenna, some of the real cheapies are particularly pathetic to start with.

As for your antenna tests, I guess that particular BNC antenna didn't work so well with your PMR446 radio, where as your home made jobbie was a more efficient radiator on PMR446 frequencies. It could be that your particular BNC antenna is designed for a wide bandwidth and just isn't too resonant on PMR446 frequencies. It could be that your particular BNC antenna is just not resonant on 446MHz at all. I've seen some BNC/SMA rubber duckies that have absolutely atrocious VSWR on their supposed resonant frequency.
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iansradios
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Post by iansradios »

ive never had a rf burn off a small 446 radio anybody touched there bike spoke whilst txing? is it the same result?
Last edited by iansradios on 19 Jun 2008, 20:45, edited 1 time in total.
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Dayglow
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Post by Dayglow »

do you think the thickness of the radiator element makes a difference.
Does a thick one require more energy to shake it so to speak?
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Post by Guzzy »

It's all interrelated.

Hit Google, there are tons of websites with info about UHF antenna design.
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iansradios
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Post by iansradios »

Try garex. pete is happy to make any spec of aerial you need.
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Dayglow
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Post by Dayglow »

Garex, now that's a name from the past, I think my dad built his first radio with bits from him. I'll have to ask.
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