testing a VHF aerial with multimeter

[Adrian]

is there any way i can test a VHF aerial with a multimeter? i want to test if the connections are OK ie that there isn’t a bad connection or break in the cable. Im having trouble with reception and want to discount any problems with the aerial before getting the radio checked.

Sorry if this is a really stupid question!!

 

[wooslehunter]

This is one that comes up time and time again. A multimeter only checks dc resistnace and not r.f. impedance. They're two completely different things. An antenna should be 50ohm but that's not resistance. It's impedance at the working frequency.

If you just use a mulitimeter, you may or may not read a dead short if you test the radio connector. Most antennas seem to read a short but that's no guarentee you have a good system since it may be a genuine short at the plug or in the cabel or antenna.

The best you can do with a mulitmeter is disconnect the antenna and simply check continuity and inner to outer shorts along the cable. This of course meens a trip up th emast with some long cables. You're only testing the cable as well and not the antenna.

The simplest way to test an antenna is to use a VSWR meter. This fits between the radio and the antenna cable. Don't use a CB radio one as they can introduce problems of their own. Get one for a few £ from an amatuer radio shop. Anything that's designed to cover the 2m (144MHz) band will also cover the marine VHF band. Look here or here ..

Simply, what they do is to give you a reading of the ratio between trasmitted power and reflected power. In a perfect system, you'll get no replecetd power so the VSWR will be 1:1. If you have a short or or an open circuit system, you'll get 1:lots. Anything less than 1:1.5 is fine and 1:2 is accepable. 1:3 meens 50% of the power you push up the cable is comming back.

 

[whipper_snapper]

It is not a stupid question but it is a (very) frequently asked question.

The simplest way to test an aerial is simply to try a different one - an emergency anteanna is a very useful piece of kit to have anyway.


Most aerials will look like a short circuit to a dc multimeter and so are hard to test. You can test the coax that feeds it - sort of - if you unplug it at both ends. You then should see continuity between both ends of the screen and the same for the centre conductor. You should see open circuit between them. If you don't, then you have a problem, but if it looks ok with a multimeter it may still be useless for radio frequencies.

 

[LadyInBed]

A resistance reading of somewhere between 100 & 600 ohms between the core and screen will tell you that your aerial and coax isn't open circuit.
Which is a bit of a help

 

[shmoo]

[ QUOTE ]
A resistance reading of somewhere between 100 & 600 ohms between the core and screen

[/ QUOTE ]

Not on my VHF anntena it won't. I has a short copper coil directly across the the core and the screen. I doubt a multimeter would read any DC resistance at all.

 

[Adrian]

thanks wooslehunter, thats exactly the sort of answer i was looking for, ill give it a try.

 

[Dave_Snelson]

me too - I'm having trouble with my new Tri-watch Cobra

 

[tome]

Correct, you'll see close to zero DC resistance which merely confirms that your coax isn't open circuit

 

[Roberto]

I just tested mine: first antenna showed 1.4 ohm resistance, I had to change it as its base was brittle, the new one shows 1.2 ohm, and tested ok on both rx/tx

Resistance values may change from antenna model to model but I think this may be a good starting point /forums/images/graemlins/smile.gif

 

[tome]

What does the resistance measure when you short the probes together?

 

[Roberto]

hello tome,
it shows zero when the two multimeter probes are connected together

actually, when I put the probes on coax shield and central wire the ohm reading goes 1.6 - 1.4 then settles at 1.2 after a few seconds

I must say it is a cheap multimeter, though it is spot on to 0.01 in determining voltage drop

anyway, I do not know how these VHF antennas are built, but imho if two of them in good working conditions show similar readings, either the reading is totally insignificant or it may mean something /forums/images/graemlins/confused.gif

 

[seacomm]

Of course the short or near short circuit may only indicate that the cable has been pinched somewhere or there is a screw or pop rivet through it at some point.

It's also worth remembering that some aerials will show a complete open circuit when their DC resistance is measured.

In reality DC checks, substitution of radio and aerial and VSWR checks are all a help but can also mislead.

There is of course a way of checking an aerial properly but, as is the way of the world it involves lots of beer vouchers spent on fancy equipment. /forums/images/graemlins/frown.gif

 

[shmoo]

My guess is the 1 point something ohms reading, if done from the radio position, is the DC resistance of about 30 meters of copper (up the core and back down the sleve, don't forget) plus that contributed by slight oxide film at any connectors. It won't be the antenna itself.