VHF aerials and the coax

[BarryH]

I've got five mins and this cup of tea and fag before I have to shoot off to the surgical spirit and ether fuelled atmosphere this afternoon. So a quickie for ya! Got a new VHF aerial at the weekend. Its got 6 mtrs of coax on it. Can I cut off the lump of coax I don't need or not. Its about 4mtrs too long. I've forgotten the maths for working out the physical length of aerials for the particular frequencies etc. Anybody know the answer to save me mucking it up?

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[longjohnsilver]

I'd just cut it, can't see any problems doing so, have done it in the past with no adverse affects, still walking, talking and hearing /forums/images/icons/wink.gif

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[wakeup]

I think it the length of th coil in the Aerial that is critical so as long as your not cutting the aerial it should be ok.

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[longjohnsilver]

Barry suspect that you're thinking of the length of a radar cable which shouldn't be shortened.

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[jimi]

What happens if you do cut the radar cable?

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[tr7v8]

Cut off the cable as required it's the aerial length that's critical not the cable.

Jim
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[longjohnsilver]

It doesn\'t work!!

Not really sure, the chap who installed mine told me it was a no no, but don't know why. He hid masses of cable in the radome and in the cabin rooflining.

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[andrewa]

I used to do radio propagation in the armed forces, HF, VHF and Sat coax has nothing to do with operational range its the height of the antenna thats important and tuning the radio to the frequency range though I think all modern sets do this for you. Just re-terminate again properly in fact I think the shorter the coax the better. It is going back a few years though.

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[BarryH]

Yep, know that the hight of aerial and line of sight and all that. What I was getting at, was back when I did my radio courses in the dim and distant passed, I remember that keeping the coax in fractions of wave made the transmiter more efficient. I other words is the 6 mtr length of coax supplied a fraction of a full wave. ie half wave, quater wave etc. so as to keep the SWR and reflected signal to a minimum.

I'll cut the damn thing and be done with it. Its not as if its a sensitive transciever. Right then wire cutters, where did I leave'em...........

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[tr7v8]

Don't forgrt you'll have to re-solder the plug as well, so look for soldering iron as well as cutters /forums/images/icons/smile.gif, I understand knotting the cable doesn't work!

Jim
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[byron]

<font color=blue>The shorter the run of coax the better, so chop away merrily.

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[byron]

<font color=blue>Yep! You don't need whistle in the mike anymore while you tune the signal {wink]

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[Bejasus]

I always thought that you adjusted the SWR to 1 or less by altering the length of the aerial & using an SWR meter.

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[BarryH]

umm, yep. You won't rea;istically get 1 to 1 ratio though. Anything connected to the ant socket becomes part of the ant. Although the coax is screened it still radiates some of the transmitted power so is part of the ant. Most marine aerials of the "whip" kind will hsve a loading coil to trick the RF side of the transciever into thinking its got an an aerial of the correct electrical length. What you really want to stop is the RF signal being reflected back into the RF circuits.

All gets rather involved if you delve too deeply. I just wanted to know wether the 6mtr lemgth of coax was a set length for the wave length of the marine bands! Too late now cos I've cut it!

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[Forbsie]

Wish you'd told me yesterday. I could have lent you my wonderful coax ratchet crimping tool, but you've probably got one anyway. /forums/images/icons/wink.gif

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[BarryH]

Coax crimping tool. Forbsie you worry me, are you a closet hairdresser. No haven't got one of those, Got a scapel tho so that'll have to do.

Did you get any further with the engine at all. Give me a shout and I'll try and be around to lend an extra pair of hands.

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[Piers]

No problem with shortening the VHF aerial coax, BUT, if you leave around 3 feet you will severly reduce the signal strength due to resonance. So, minimum should be around 4 feet.

Keep copnnectors to a minimum. You lose some 3dB for every connector used.

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[qsiv]

Ah - but you're not expecting to radiate RF from the coax feed (it's shielded for one).

What does matter is tuning the radiating part of the aerial to the frequency range you will be transmitting / radiating on.

UHF (Radar) is much more picky - even things like cable bends must be within limits. I'm surprised though that his applies to marine leisure radars - they dont tend to have the magnetron in the set, it's usually part of the scanner array - so the only reason for not shortening is the monster number of cables that are fed up to the radome. Big ship radars otoh have separate magentrons, and the microwaves are fed up wavguides to the scanner - they have to be shielded (by a cage) and touching one will fry you - thats why the leisure marked doesnt use them - also the power of leisure radars is so low that it is easy to put the magentrons in the scanner.

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