Re: Measure VHF Antenna (Cable) ?

"Chris" <Chris_MdR@xxxxxxx> wrote in news:1142183322.633964.220320
@i39g2000cwa.googlegroups.com:

I suspect that the cable leading to the antenna is bad,
disconnected, or whatever. (Does this sound like the likely
cause?)



Everyone with a VHF radio needs:
http://www.buyreliant.com/marine/accessories/art2.htm
mounted right next to its main transceiver.

It goes between the radio and the antenna and the RF energy powers it.

There are 3 positions...POWER...SET...SWR

Power is a simple diode power meter that shows, close enough, what power
the radio transmitter is putting out when you press the button. The TX
light on the radio does NOT mean anything is coming out of it. The meter
does.

To test the antenna, you put the switch in the SET position, key the
microphone (off Channel 16 please!) and turn the little set adjustment
until the meter reads full scale. Then, while still transmitting, flip
the switch to SWR and PRAY the meter reads nothing, a perfect antenna.
This is the power reflected by the imperfect antenna match up the mast.
They all read "something" reflected. If the cable is open or shorted or
the antenna is zapped by the lightning hit last week, the meter will read
HIGH, moving way up past half scale (3:1 on these little meters). The
SWR reading in the lower half of the scale, like 1, 1.5, 2, 3 (to 1)
shows how well the antenna is tuned. 1 means all the power you are
transmitting, minus a little loss in the cable, is going out on the air.
1.5 means 4% is reflected back. 2:1 means 10% is reflected back, 3:1 is
25% reflected back and wasted. Any reading below 2:1 is fine...it'll
work great. 3:1 is very marginal, something is wrong that needs fixing.
Most antennas are around 1.5:1. Test a few channels from the lowest to
the highest you use. The antenna has a bell curve of SWR that should be
higher on the end channels and lower in the middle, its tuning range. If
the low channels are lower and the SWR rises up as channel numbers
increase, the antenna element is too long, the bell is below the marine
band. If the opposite is true, the high channels lower and SWR rises
towards the low end, the antenna is too short and needs lengthening. If
you have a fiberglass antenna, and it tunes too low, it means the antenna
is too close to some metal object or other antenna, lowering its resonant
frequency by capacitance effect. If it's high, tough luck. It's not
tunable like a Metz Manta's little sleeve nut allows.

To mount the stupidly-designed meter, go to Radio Shack and by two right
angle RF connectors with the male on one end and female on the other.
This will route the RF back through two holes in the expensive mahogany
panel to the cables behind. Surface mount the meter between the holes
you drilled that don't quite fit even...(c; The meter uses no measurable
RF power from the transmitter, so leaving it in line so it reads all the
time doesn't cost you anything. It won't wear out. Knowing FOR SURE
your distress calls are FOR SURE going out on a GOOD ANTENNA is easily
worth $42 and the hassle of installation. Don't forget to buy the little
jumper to go between the meter and radio...

Waste Marine, Boat/US, Boater's World, and the rest all carry this
Shakespeare meter. Caveat Emptor

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