(Tom Bruhns) wrote in message m...
(Dr. Slick) wrote in message . com...
...
You're right about this, and it reminds us that if there is any
loss at all, we theoritically move away from a purely resistive
characteristic impedance into a complex one. This furthers the
complexity on the problem, as we must expect reactance in our coax.

It's not just theory, it's practice and measurable. But it's not a
requirement that loss introduce reactance to Zo; it's just that if the
line is lossless it must have a non-reactive Zo. Clearly if L/C =
R/G, the impedance will be non-reactive.


True. Never considered this before, thank you. Makes sense that
if the ratio of the series resistance and shunt conductance are the
same as the ratio of the series inductance and shunt capacitance, that
the transmission line will still be non-reactive.


Also, consider what reactance you do get in practical lines, at what
frequencies. What effect does frequency have on the reactance? Why?
Under what conditions will it really be a problem? Might the reactive
part be so small that it will be totally swamped out by variations in
the real part? Think a bit about how much it will or won't mess up
the measurements you want to make.

Cheers,
Tom

In general, using an MFJ antenna analyzer to get a rough idea of
what the series equivalent complex impedance is (these are not $80,000
vector network analyzers!), it seems that cheap 3' RG-8x jumper coax
cables mainly add series inductance to the system, as the reactance
gets higher with increasing frequency.
We try to design 9 element chebychev low-pass filters, which is
not difficult at very low power levels, as two 1/4 watt 100 Ohm
resistors make a decent dummy load all the way out to 200 megs or so.

The problem is characterizing insertion loss using higher power
transmitters, when we know that the 1000 watt cantenna swings from 40
to 70 Ohms (with reactance too) as you get above 80 megs or so. It
become difficult to know if you are moving in the right direction or
not.

Slick

(Dr. Slick) wrote in message om...

The problem is characterizing insertion loss using higher power
transmitters, when we know that the 1000 watt cantenna swings from 40
to 70 Ohms (with reactance too) as you get above 80 megs or so. It
become difficult to know if you are moving in the right direction or
not.

So if you have 200 feet of RG-213 and 200 feet of RG-58, put those in
series to the cantenna. Coil them loosely and cool them with a fan if
needed, if you are running high power (or start the chain with larger
coax). That should get you close to 15dB attenuation in the coax at
100MHz and a 1.02:1 or better SWR at the input end for a 2:1 SWR at
the cantenna. At lower frequencies where the line loss is lower, the
cantenna is good enough that you don't need the line loss as
compensation. Do I have you worried about what the actual impedance
of the coax is from earlier postings? If so, good! Now you're in a
position to think about what's good enough, and measure what you have
to see if it meets the desired goals.

If that's all too kludgey for you, go buy a good load. Or just tune
the cantenna for bands of interest. You could make a set of small
boxes with L networks, each of which compensates the cantenna on a
particular band. Or look up one of the articles about how to make a
better "cantenna", probably starting with the parts you have. There
are lots of options. Why settle for a setup which will only lead you
deeper into confusion? Short of really precision measurements,
there's a lot you can do trading off time and careful thought for the
high cost of commercial equipment.

Cheers,
Tom

Tom Bruhns wrote:

So if you have 200 feet of RG-213 and 200 feet of RG-58, put those in
series to the cantenna. Coil them loosely and cool them with a fan if
needed, if you are running high power (or start the chain with larger
coax). . .

One thing to keep in mind when you use coax as an attenuator or dummy
load is that the portion of the cable nearest the transmitter dissipates
most of the power. If you had, say, 6 dB per 100 ft attenuation and a
200 ft cable, the first 50 ft dissipate 1/2 the power (and that's
concentrated toward the transmitter end), the next 50 ft dissipate 1/4
the power, the next 50 ft 1/8, and the final 50 ft 1/16. So do as Tom
says and put the heavier coax up front, and allow for more air
circulation for the coax nearest the transmitter if dissipation becomes
a problem.

Roy Lewallen, W7EL