Testing the vhf cable to see if the aerial is good working condition

Scotty_Tradewind

Scotty_Tradewind

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With basic gear such as a multimeter is it posssible to test accross the wires at the radio end to see if I have a good, efficient aerial?

A good friend now departed to a better place used to test my aerial each winter on my old boat with a special meter. Now he's sadly not with us I have no idea what this little gadget was or what it did.
Any offers?
 
The correct device for testing aerials is an SWR (Standing Wave Ratio) meter - you need one that's for marine band VHF, not CB radio. You connect it between the transmitter and the aerial.
 
Scotty_Tradewind said:
http://www.google.co.uk/#hl=en&xhr=...gc.r_pw.&fp=ffb2143cbdee9237&biw=1280&bih=785

ThANKS FOR THAT
Are they simple to use.... any suggestions which one?
Click to expand...

I lend mine out for an RNLI donation - but in season, that is Greece only!

Very easy to use. Link into aerial lead; switch to "Reference"; press transmit button and adjust scale to zero with knob on front; then switch meter to "measure" and transmit again. If needle stays in green section (SWR = 1-1.7) then all is OK. Mine is marketed by Shakespeare, the aerial manufacturers.

Chas
 
Without wishing to offend the purists, may I say that I have had perfectly acceptable results using on old CB "swaarmeter".
The actual power reading may not be accurate at frequencies so far removed from the designed band, however, it's a RATIO we are looking for, so, having set the reference voltage to a value the meter 'thinks' is "1", we get a comparison to that when we switch to reflected.

A quick way of checking that the VHF is transmitting at full power ( it will reduce power automatically if it senses antenna problems) is to look at the boats ammeter while transmitting.
It should draw about 5 amps running at 25watts. Significantly less than that means it's in "protect mode".
 
It's possible to get a 'good' reading when the system is bad. For instance, if the braid has corroded. This is because the meter measures the ratio between the signal going up and the signal reflecting back - if the signal going up is low due to a corroded braid, the ratio can be erroneously in the 'good' range.
The SWR meter is no substitute for checking for bad connections and corroded cable. If those issues are OK, then the SWR meter will tell you if the antenna is OK or not.
 
Salty John said:
The SWR meter is no substitute for checking for bad connections and corroded cable. If those issues are OK, then the SWR meter will tell you if the antenna is OK or not.
Click to expand...

Well, at least it'll tell you that the aerial is "safe" to use - the real test is to transmit successfully over a good distance. You can receive with a bent coathanger, but clear, strong transmission require a good aerial. It is only sensible to inspect first for curable faults, check its safe for the radio (SWR reading) and then final test (call a friend).

Rob.
 
The OP talks about an annual check. This seems excessive? I'm a big believer in the "if it ain't broke, don't fix it" philosophy. It's always wise to regularly check vhf output signal by calling a friendly boat on the horizon. Anyone recall RST? Always nice to receive in morse abbreviated as 5nn.

I could imagine that getting to the back of the set and removing the antenna connection once a year risks collateral damage somewhere around the panel, obviously depending upon access.

A cheap CB SWR meter is more than adequate for the job. Simply set the radio to low power, adjust outpout meter to full scale deflection and read the refelcted vswr. If it's below 2:1, it's probably fine. That said, a low swr can be obtained with other problems in the system. In my opinion, it's best used for initial commissioning or as part of a problem diagnosis procedure, rather than annual health check.
 
davidjackson said:
The OP talks about an annual check. This seems excessive? I'm a big believer in the "if it ain't broke, don't fix it" philosophy. It's always wise to regularly check vhf output signal by calling a friendly boat on the horizon.
Click to expand...

Depends if you take the mast down every year. I do, and the process involves removing the connector from the aerial to get the cable through the deck gland. The reverse process in the spring means refitting the connector, and this sometimes leads to problems.
 
Salty John said:
It's possible to get a 'good' reading when the system is bad. For instance, if the braid has corroded. This is because the meter measures the ratio between the signal going up and the signal reflecting back - if the signal going up is low due to a corroded braid, the ratio can be erroneously in the 'good' range.
The SWR meter is no substitute for checking for bad connections and corroded cable. If those issues are OK, then the SWR meter will tell you if the antenna is OK or not.
Click to expand...

good point

Chas
 
Salty John said:
It's possible to get a 'good' reading when the system is bad. For instance, if the braid has corroded. This is because the meter measures the ratio between the signal going up and the signal reflecting back - if the signal going up is low due to a corroded braid, the ratio can be erroneously in the 'good' range.
The SWR meter is no substitute for checking for bad connections and corroded cable. If those issues are OK, then the SWR meter will tell you if the antenna is OK or not.
Click to expand...

Re my previous post - if the "signal going up" is below spec,(for whatever reason) the ammeter will register that fact.
 
Checking aerial

A simple check of the antenna and cable is to measure the DC resistance from outer to centre of the plug at the radio. This will often be quite low resistance being the DC path up the cable through the antenna and back through the braid. Hence any break in connections etc will show up.
Problems are that not all antennae have a DC connection from centre to earth. And of course this check will show OK if there is a short between centre and braid.
So check the DC path when you know the radio/antenna is good or mast is down(antenna disconnected and connected) and write it down for future reference.
Yes a VSWR meter can give readings that are misleading. ie mediocre when really bad.
The meter measures power going out compared to power reflected back (from a bad antenna). Bad cable or even good cable of poor quality can attenuate the power in both directions.
good luck olewill
 
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