Doc Johnson wrote:
> I am not sure where to ask this question but it kinda fits in this forum. I
> have a 2 meter repeater system. It consists of multiple RX feeding a voting
> system then feeding a 375 watt Micor xmitter. The xmitter is located on top
> of a building approx 60' away from the antenna (DB224 4 bay dipole) which
> is fed with 7/8" foam hardline. I am using a 2' jumper to connect the
> xmitter to the antenna which has been 9913 and rg213. The jumper gets
> quite warm to the point it is uncomfortable to touch. Another confusing
> issue is I tune the amp up with a Bird watt meter and when I remove the
> watt meter the plate current lowers by about 50-75 mills. Any input? SWR
> is 1.3.:1 which seems a touch high for an antenna that is tuned for the
> amateur band.
>
Hi Doc,
First be sure the antenna is tuned for the ham band. Most commercial antennas
will not cover the 2 meter ham band. The antennas are broadly advertised
sometimes to "cover 144 to 174 mhz". But it may take 2 or 3 different antennas
to cover that range. A 10 mhz bandwidth on those antennas is not uncommon. If
it was a 150 to 160 mhz antenna it will not work well on 2 meters at all.
As far as measuring the reflected power, you should make an additional coax
jumper to be used inline with the meter so that the meter plus the extra line
makes a 1/2 wave length. Then when you put the meter inline the transmitter
will see the same antenna impedance that it sees without the meter inline. I
think bird tells you in the manual how much length to figure for the meter
itself, as the line section in the meter is an air line and has a different
velocity than coax. Also move the meter from one end of the jumper to the other
and note the difference in reflected power. You may have more reflected power
than you are reading if the antenna is badly mismatched.
73
Gary K4FMX
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