SWR meter differant impediances
 

I have a 50 ohm swr meter (Heathkit, Drake W4 and Bird) and I am using 70
ohm coax for a dipole antenna on 3.7 MHz.
If I trim the antenna for the lowest reflected power showing on the meters,
will the antenna be cut to resonate (or near resonate) frequency or is it
possiable the antenna will end up around 100 feet or 150 feet instead of
near a calculated frequency length of around 125 to 130 feet.

Guess the real question is can I use a 50 ohm meter with 70 ohm coax and
when the reflected power is the lowest, is the antenna system adjusted
correctly ?

I don't really worry about it, just stick up some wire and operate. Adjust
things so the rig will see a low enough swr the internal tuner will take
care of it.

SWR meter differant impediances
 

Ralph Mowery wrote:
I have a 50 ohm swr meter (Heathkit, Drake W4 and Bird) and I am
using 70
ohm coax for a dipole antenna on 3.7 MHz.
If I trim the antenna for the lowest reflected power showing on the
meters,
will the antenna be cut to resonate (or near resonate) frequency or
is it
possiable the antenna will end up around 100 feet or 150 feet instead of
near a calculated frequency length of around 125 to 130 feet.

Guess the real question is can I use a 50 ohm meter with 70 ohm coax and
when the reflected power is the lowest, is the antenna system adjusted
correctly ?

I don't really worry about it, just stick up some wire and operate.
Adjust
things so the rig will see a low enough swr the internal tuner will take
care of it.


When your SWR meter reads 1:1, it means only that the impedance it sees
is 50 + j0 (50 ohms purely resistive). If you're using 70 ohm line, the
impedance at the antenna could be any of an infinite number of different
impedances, depending on the length of the coax. It might or might not
be resonant, but that won't make any difference to its performance. Your
transmitter will be happy with the load, so the antenna will get the
full rated output power less coax loss. It would be appropriate to call
that the "correct" adjustment of the antenna.

If the tuner has to adjust for some other load impedance (indicated by a
higher reading on the SWR meter), the tuner loss will be added. However,
it's likely to be negligible if the SWR meter is reading 2 or 3:1 or lower.

Roy Lewallen, W7EL

SWR meter differant impediances
 

"Ralph Mowery" wrote in
:

I have a 50 ohm swr meter (Heathkit, Drake W4 and Bird) and I am using
70 ohm coax for a dipole antenna on 3.7 MHz.
If I trim the antenna for the lowest reflected power showing on the
meters, will the antenna be cut to resonate (or near resonate)
frequency or is it possiable the antenna will end up around 100 feet
or 150 feet instead of near a calculated frequency length of around
125 to 130 feet.

Guess the real question is can I use a 50 ohm meter with 70 ohm coax
and when the reflected power is the lowest, is the antenna system
adjusted correctly ?

I don't really worry about it, just stick up some wire and operate.
Adjust things so the rig will see a low enough swr the internal tuner
will take care of it.

Ralph,

Your 50 ohm VSWR meter indicates the VSWR that you would expect on a low
loss 50 ohm cable adjacent to the meter.

For indicated VSWR1, there is an infinite set of impedance values that
would satisfy the indicated VSWR, and a further infinite set of VSWR
values that those impedances would imply on the adjacent 70 ohm line.

If the indicated VSWR was exactly 1, then the impedance at that point is
50+j0 and the VSWR on the 70 ohm line is 1.4.

Now there is a possibility!

My 40m dipole is fed with RG6 coax (75 ohm), and a bunch of ferrite
sleeves in the style of a W2DU balun at the feed point. I have used a 75
ohm VSWR meter in that line to adjust (shorten) the dipole length until
VSWR=1.5. Now, the impedance at any point along the line varies with
displacement, but at some points, the impedance will be R=75/1.5, X=0, Z=
50+j0. Having approximately calculated a convenient length, I have cut
the line 2m longer, inserted a 50 ohm VSWR meter at the tx end and
trimmed (shortened) the coax until the VSWR(50)=1:1. There it is, a
shortened half wave dipole, fed with inexpensive coax, very low line
loss, and an excellent 50 ohm load for a modern transceiver. This is of
course a single band solution, but it is an optimal single band solution.

The example above indicates that you could see a perfect VSWR on your 50
ohm meter. You are unlikely to find the solution by chance alone, but
taking a systematic approach, you can find the solution.

Owen

SWR meter differant impediances
 

Ralph Mowery wrote:
I have a 50 ohm swr meter (Heathkit,

If it's the HM-15, you can simply swap out the
50 ohm resistors with 75 ohm resistors and have
a 75 ohm SWR meter.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com