Hi Paul, I think you are confusing impedance and resistance here. There is
an infinite number of resistance and reactance combinations that will equal
50 ohms impedance but the only ones with low vswr are the ones at or near
50 +j0.
Interestingly, a coax long enough to have substantial loss does make a good
dummy load. For example, a 100ft length of RG213 has about 10 dB loss at
432 MHz. With a short or open circuit at the far end, the reflected wave
will be attenuated 20 dB (this is what 20 dB return loss is all about). A 20
dB reflection is equivalent to 1.2 vswr. When the length is doubled, the
return loss is 40 dB and vswr is like 1.02 or so.
I use a length of RG213 like this for a dummy load on 432, 1296 and 2304
MHz with excellent results. The only caution is that the coax gets really hot
when it is absorbing 1500 watts power at these frequencies. After just a
few minutes there is a smell of burning plastic and you know it is time to
let the cable cool a bit.
Maybe a bucket of water would take care of that but I have never tried
that. Taking all this one last step, one could monitor the temperature rise of
the bucket of water to make a poor ham's calorimeter for precision power
measurements.
73,
Gerald K5GW
In a message dated 2/2/2010 7:50:04 P.M. Central Standard Time,
w9ac@arrl.net writes:
> Which is why W2VJN's stub cookbook shows some interesting behavior
> with 1/8 wave connecting lines <G>.
Using TLW a while back, I was modeling a 1/8wave line and it looks like
no
matter what the termination Z at the end of 1/8wave line, the Z seen at
the
transmitter is always the line Zo. Depending on whether the line is open,
a
short, or somewhere in between, the magnitude of reactance varies greatly
as
does the line SWR, but the Z seen by the transmitter will always be fixed
at
Zo.
Hypothetically, a length of say...RG58 in odd multiples of a 1/8wave
would
make a great dummy load on paper with exceptionally high return loss when
the termination is either a short or open. However, I'm not sure how
various transmitters would take to a Z of 50 ohms where the reactance
component radically changes between an open or shorted termination but the
power dissipating resistance portion is only a few ohms.
Paul, W9AC
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
