VHF Antenna Resistance

[wicked]

Hi, can anyone tell me what resistance I should be getting at the plug on the end of the VHF cable, measuring between the core and the sheath ?. I'm getting about 1 ohm, which I guess indicates a short, possibly at the deck connection. Should I be getting 50 ohm?

Also can I use a CB type SWR meter for marine VHF ?

Thanks.

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[silverseal]

I think it is 75 ohm for coax. 1 ohm is too low- dont use transmitter until sorted or you will blow the output transistors

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[bedouin]

You cannot effectively measure the impedance of an aerial using a DC ohm meter. A DC resistance of 1 ohm does not sound impossible. The characteristic impedance of 50 ohm relates to it's impedance at radio frequencies.

Marine VHF operates around 160MHz, so you need an SWR meter that covers that frequency. IIRC CB operates at about 27MHz, so a CB SWR meter may or may not cover the marine band - it will depend on the meter itself



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[MainlySteam]

The resistance, as you have measured it, may be either a short circuit (as you basically have) or an open circuit depending upon the electrical design of the antenna. Most commonly they are short circuit as fitted to sailboats.

As others have pointed out, the resistance you have measured has nothing to do with the 50 ohm characteristic impedance of the coaxial cable you are using (nor that of the antenna either, which may also be described as being "50 ohms" that being the feedpoint resistance of the antenna at radio frequency).

I would not use a CB SWR meter, for the reason that they are junk, and because the nature of your original question indicates that you would not correctly interpret what you were seeing (that meant kindly as the comment applies to almost everyone).

John

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[William_H]

Yes a CB type VSWR meter should work fine. I used one proffessionally on aircraft for many years at 120 mhz. See later post re emergency antenna to try as substitute.

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[LadyInBed]

Today I measured the resistance across a new VHF Antenna (no coax attached) - 1 to 2 ohms and my old emergency antenna plus about 15 feet of coax - 4 to 5 ohms.

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[Geordie]

It is in fact a DC short circuit, of a very precise physical length. At VHF frequencies it is not a short circuit.

This is in fact a useful way of checking that your connections are correct, but it is not a check that your antenna is OK. A VSWR meter is required for that.

<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by Geordie on 19/08/2004 22:32 (server time).</FONT></P>

 

[bruce]

without antenna attached, there should be no reading. not knowing what antenna you are using, can only recomend taking antenna off mount and cleaning crud off and checking insulators etc, and add sil grease before re mounting as well as mount to boat attachment. gen antennas do not go bad without some form of damage to them but too much power on a cheap antenna has been known to burn the loading coil or cracks in l/c due to age. a check of center pin to antenna body shoule be direct, and there should be a reading between center and ground equal to antenna such as 50 ohms or what ever you have. if just a simple whip with no loading coil there will be no reading on coax between inner pin and outer shield unless grounding in the mount has failed. they fail from cracking due to being too tight or oldage/weather.

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