clyst
Well-Known Member
Sorry for a numpty question but I think my VHf for aerial may not be fully working (receives ok but iffy transmission). Presume there is a simple way to static test with a multimeter .
Cheers
Cheers
How to test whether a VHF aerial is duff?
- Thread starter clyst
- Start date
ghostlymoron
Well-Known Member
I don't think a multimeter will tell you much. You need a SWR. Probably better to borrow than to buy. Aerial problems are often more to do with the cable than the aerial itself and you can test that with a multi.
VicS
Well-Known Member
Only some basic tesst you can do with a multimeter.
If you have a deck plug unplug the aerial and the lead from the radio.
Check with the highest ohms range that there is no circuit between the inner and outer of the cable between set and deck socket. Then short the two together at the deck socket and check gain using the lowest range or continuity buzzer if you have one. You should see zero ohms.
Now turn you attention to the lead from deck socket to aerial. Check with a low ohms setting. If you do not see a very low resistance be suspicious of the aerial and /or its lead.
If you can borrow an SWR meter do so and check with that
The ultimate test is to try a known good aerial ..... the emergency aerial packed with your flares for example.
Check also the power supply to the radio while you are attempting to transmit. The radio draws little current when in receive mode but several amps when transmitting
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PetiteFleur
Well-Known Member
I hired a swr meter from Fox's in Ipswich - from memory it was about £10 which was shared between 3 boats.
Baddox
Well-Known Member
I had a similar problem and bought a SWR meter for under £20 from the auction site. It worked well enough to diagnose that the aerial/coax needed replacing. You’re welcome to borrow it if you’re close.
nigelmercier
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You can get a SWR meter for about £30, handy bit of kit: http://amzn.to/1zCOpac
Billjratt
Well-Known Member
Before you get an SWR meter, try this stuff. If the boat doesn't have an ammeter, use your multimeter to measure the current drawn by the VHF from the battery . (set to DC AMPS, high range, in series with the fuse.) If you transmit at high power (25watt) it should draw about 5amps if the set develops full power.
If it doesn't, the VHF may be reducing power to protect itself from reflected power due to an aerial/cable problem. It may transmit properly on low power with a small 1amp-ish, measurement on the meter.
What does it do on the emergency aerial? Same measurements will give an idea of what should happen. If you don;t have an emergency ae, borrow/get one...
If the indications point to an aerial problem, check and repair the deck plug/socket first. Most marine aerials measure as a short circuit on a multimeter so either a SWR meter or scope and TDR process are required if further investigation is needed. SWR will say there's a problem TDR will show where/what it is.
If it doesn't, the VHF may be reducing power to protect itself from reflected power due to an aerial/cable problem. It may transmit properly on low power with a small 1amp-ish, measurement on the meter.
What does it do on the emergency aerial? Same measurements will give an idea of what should happen. If you don;t have an emergency ae, borrow/get one...
If the indications point to an aerial problem, check and repair the deck plug/socket first. Most marine aerials measure as a short circuit on a multimeter so either a SWR meter or scope and TDR process are required if further investigation is needed. SWR will say there's a problem TDR will show where/what it is.
clyst
Well-Known Member
Thanks for all your replies chaps ,very helpful. I think anSWR meter is the answer ......... Just like my ol' CB days !!
Cheers all .
Cheers all .
wooslehunter
Well-Known Member
But don't use a CB meter. Make sure it's OK for marine band. Ham 2m is close enough (145MHz ish) to make no difference. A CB Meter may be fine but may not.
Billjratt
Well-Known Member
CB SWR meter should be OK. remember it's a ratio you're looking at - the difference between reflected and forward power. It doesn't matter whether the actual wattage is accurately displayed or not, as long as you see something.
William_H
Well-Known Member
Firstly a VSWR meter can be misleading if you insert it at the transmitter and yu have a long lead to the antenna of thin coax. In the case of a good antenna the power goes up but does not come back. In the case of a bad antenna the power is reflected back to the transmitter. The VSWR mter measures the ratio of power up to power down. The problem being that with a long lossy coax the reflected power can be reduced (lost) in the losses of the coax (making a return journey)
So even with no antenna connected a 15 metre cable of lossy coax (common RG58 6mm diameter) can indicate that only half the power is coming back. Now a good antenna with no loss in the cable might show 1/4 of power coming back. So a good VSWR less than 5% return might indicate a good antenna a 25% return might mean a bhad antenna or maybe not. To properly check an antenna VSWR you need to test from close to the antenna. (not conveneient)
Now VHF antenna for a boat come in 2 styles. The first simple is a stainless steel rod about .5 metre long and insulated from its mount where the centre of the coax cable connects to the rod and the outer connects to a large area ground like the mast. This antenna should always show high DC resistance from centre to outer. Continuity of the whole cable can be checked by earthing the antenna for the test.
The next knid of antenna is a bit more tricky. Typically being inside a fibreglass tube and 1.5 metres long the cable goes inside the tube and there is no connection to earth (the mast.) This kind are often described as having "gain" over a 1.4 wave dipole (the first type.) They usually have a connection to a coil inside such that it is correct for there to be a DC connection outer to inner of the feed cable.
So this won't help OP but it is useful to check a known good antenna system with a multimeter on ohms from the trnamitter end of the cable to know what to expect if later you have doubts.
So best in practical terms is the ohmeter test but will require access to the antenna itself wheras a VSWR meter can check from the bottom only. But again best if you know what to expect from the antenna when it is good. If you have a digital amp meter so can check the current drain of the transmitter on high power again this can be a good indication but really best again if you know what is normal good. good luck olewill
So even with no antenna connected a 15 metre cable of lossy coax (common RG58 6mm diameter) can indicate that only half the power is coming back. Now a good antenna with no loss in the cable might show 1/4 of power coming back. So a good VSWR less than 5% return might indicate a good antenna a 25% return might mean a bhad antenna or maybe not. To properly check an antenna VSWR you need to test from close to the antenna. (not conveneient)
Now VHF antenna for a boat come in 2 styles. The first simple is a stainless steel rod about .5 metre long and insulated from its mount where the centre of the coax cable connects to the rod and the outer connects to a large area ground like the mast. This antenna should always show high DC resistance from centre to outer. Continuity of the whole cable can be checked by earthing the antenna for the test.
The next knid of antenna is a bit more tricky. Typically being inside a fibreglass tube and 1.5 metres long the cable goes inside the tube and there is no connection to earth (the mast.) This kind are often described as having "gain" over a 1.4 wave dipole (the first type.) They usually have a connection to a coil inside such that it is correct for there to be a DC connection outer to inner of the feed cable.
So this won't help OP but it is useful to check a known good antenna system with a multimeter on ohms from the trnamitter end of the cable to know what to expect if later you have doubts.
So best in practical terms is the ohmeter test but will require access to the antenna itself wheras a VSWR meter can check from the bottom only. But again best if you know what to expect from the antenna when it is good. If you have a digital amp meter so can check the current drain of the transmitter on high power again this can be a good indication but really best again if you know what is normal good. good luck olewill
lw395
Well-Known Member
There are some CB SWR meters which don't work at all well at 170MHz, being designed for 27MHz.
Others work fine.
What you need is the HP8753 on my bench, probably cheaper to just buy a new aerial TBH.
At the end of the day, a radio check with a moderately distant station on low power tells you a lot.
