Testing VHF Aerial with a CB Radio SWR meter

superheat6k

superheat6k

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Does a SWR meter have to be matched to the operating frequency band.

I have an old CB Radio SWR meter - is that sufficient to check the SWR on my Marine VHF aerials or is the operating frequency of CB around 27 MHz too far from the Marine VHF channels at about 56 MHz for the CB SWR meter to be effective / reliable ?
 
Essentially the CB VSR meter is not suitable for checking your marine VHF at about 156 mhz. However if you reckon your vhf antenn is working OK it would be worth checking it with the CB meter just to see what results you get. Expect the reading to be high in forward direction the higher power and frequency giving more indication. (turn the adjustment way back and or test on low power o/p) The reflected power may show a useul small amount returned (good) but also may be a lot which may be unreliable. I would say that if the antenna is suspect and you use the meter which indicates bad antenna then it may simply mean the meter is not suitable or the natenna is bad. The results may be useful if at a later date the antenna is suspect.
A VSWR meter essentially compares power going up to that reflected back down the cable. High power up and low coming back is good. A bad antenna either shorted or open at the cable connection at the base will reflect all power back a good antenna radiates all the power and none returns.
Do bear in mind that a long coax cable especially the RG 58 (6mm aprox diameter) cheap cable will waste a lot of power in a long run from transmitter to antenna. If you have a bad antenna this dissipation of power occurs going up as well as back. So if you lose 1/2 power typical of a run of say 30 meters then if the antenna is bad then the returned power will be halved again so only 1/4 power returned which might be interpretted as a poor but not really bad antenna.
So more accurate actual antenna testing should be done near the antenna. All very complicated (or variable) good lcuk olewill
 
I have been given to understand that VSWR meter is matched to the frequency band, so you will get inaccurate results with a CB unit - but I don't know which way the results will be out! As olewill says, it could be used as a comparator by getting results on a good installation and then testing your own. I'd ask around to see if any of your friends and workmates are radio hams. They may well have a 2 metre VSWR meter that you could borrow. This is the band adjacent to the marine frequencies and will give more than acceptable results. You may also be able to pick one up fairly cheaply on ebay if you're lucky.

Rob.
 
I found swr meters generally work fine at higher freq. However the power reading can be total nonsense. But FWD/REF power is still in ratio with each other even if its not showing the correct FWD/REF power and thus the SWR is still valid.

Due to losses in coax you should test at the antenna end if possible, as per post above.
 
I found swr meters generally work fine at higher freq. However the power reading can be total nonsense. But FWD/REF power is still in ratio with each other even if its not showing the correct FWD/REF power and thus the SWR is still valid.

Due to losses in coax you should test at the antenna end if possible, as per post above.
 
superheat6k said:
Does a SWR meter have to be matched to the operating frequency band.

I have an old CB Radio SWR meter - is that sufficient to check the SWR on my Marine VHF aerials or is the operating frequency of CB around 27 MHz too far from the Marine VHF channels at about 56 MHz for the CB SWR meter to be effective / reliable ?
Click to expand...

What is the purpose of the test? Are you investigating a specific issue / loss of performance or simply want to check out of interest? If you have an SWR meter, why not use it? Most importantly, low power first!
 
davidjackson said:
What is the purpose of the test? Are you investigating a specific issue / loss of performance or simply want to check out of interest? If you have an SWR meter, why not use it? Most importantly, low power first!
Click to expand...

It was just out of interest. I do not like calling up the coast guard for a radio check and prefer a live test to be calling up say the marina on Ch 80. If they respond then at least I know the short range is OK.
 
nigelmercier said:
And if you do, don't use CH16: http://www.ybw.com/forums/showthread.php?400334-vhf
Click to expand...

I like to think I have held my VHF (Restricted) Licence for 26 years because I do know how to operate it, use correct voice procedure and know what the various channels are for and when / when not to use them. However, I will admit to finding DSC a complete PITA, too many clever functions too infrequently used to be able to remember the proper procedures.

Over and Out Roger !
 
The anser to you question is that it may be OK. Sorry, can't do any better than that.

VSWR meters are essential wide-band devices & not tuned to a band. There are various methods that can be used to sense the forward & reverse power. Some do not work well at low frequencies while some will work fine from a few MHz right up to & including marine band. Often the VSWR reading will be fine but the actual forward power will be way out.

However, CB meters are often cheaply made & putting one in the line can actually cause a poor VSWR at marine band frequencies. That's not to say every CB meter will be poor. And, price is not necessarily an indicator of performance at marine frequencies. The only real way you can tell if your meter is any good at marine band is to compare the results with a good quality one - borrowed from a friendly ham if you know any. In fact, if you put both in the line with the good quality one before the CB one, you can see how the CB meter degrades the VSWR.
 
There are several circuit topologies for SWR bridges, the transformer based ones can be inherently broad band, others are more narrow band.
But for fairly high powered, 'consumer/amateur' equipment, I would not be at all surprised to see inaccurate results when working at 5x the design frequency.
In RF, octave bandwidth (2:1) is good for many circuits.

However, if it helps you find a fault, without blowing up your transmitter, why not try it?
I would think most modern VHF sets are hard to blow up through poor SWR, but I can recall seeing 'don't operate with antenna disconnected' in manuals.
An out of band SWR meter could look just like a missing antenna or a short.
So maybe stick to low power...
 
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