wrote in message
ups.com...
If the antennas are to be spaced more than a 1/2 wavelength (free
space), how can I use a 1/4 wavelength of 75 Ohm coax? I guess I would
have to splice in a 1/2 wavelength multiple of 50 Ohm coax between the
75 ohm coax and the T connection to make up the length.

I only need about 500Khz of bandwidth, so this might not be a concern.
But this was another question I was going to raise. Is there anyway to
calculate the bandwidth that I would see from a 1/4 wavelength
matching section? I guess that is a real easy one to test in real life
though.

Thanks for all the responses!

-Scott, WU2X


Hi Scott

If it is OK to assume the antenna's input impedance is close to 50 ohms.
Any length of 50 ohm line can be installed to a location where the 1/4 wave
transformers can be installed.
It may be confusion for me to add that I have connected two 50 ohm coaxes
in series to make them look like 100 ohms at the junction. I didnt think
the joining of coax center conductor to the outer conductor of another was a
good idea till I did it (after being advised by Mr Richard Clark).
Series-ing coax works quite well at 2 meters when a couple ferrite sleeves
are used to minimize currents on the outside of the shield. That allows
only one 75 ohm matching transformer (which isnt much of an advantage in
your case).

Jerry

Oh yes, what was I thinking. From the antenna feedpoint, I can run two
equal length pieces of 50 ohm coax to the 1/4 wavelength matching
sections into the T connector - as you just said.

But it will be much more convenient for me to just use an entire run
of 75 Ohm coax, but as I said before, now you have me curious about
the effect on bandwidth - I will have a total of 1.75 wavelengths of
75 Ohm coax per leg.

-Scott, WU2X



On Mar 8, 2:31 pm, "Jerry Martes" wrote:


If it is OK to assume the antenna's input impedance is close to 50 ohms.

wrote in message
ps.com...
Oh yes, what was I thinking. From the antenna feedpoint, I can run two
equal length pieces of 50 ohm coax to the 1/4 wavelength matching
sections into the T connector - as you just said.

But it will be much more convenient for me to just use an entire run
of 75 Ohm coax, but as I said before, now you have me curious about
the effect on bandwidth - I will have a total of 1.75 wavelengths of
75 Ohm coax per leg.

-Scott, WU2X


Hi Scott

How much do you like Smith Charts?? It would be very easy to plot, for
three frequencies, the impedance at the "far end" of the 75 ohm lines.
That seems like a 5 minute job. Then you will know the amount of error
introduced at the edges of your bandwidth. You'd know the impedance at the
upper, mid and lower frequencies. It may be that a couple wavelengths of 75
ohm line may not introduce enough error to be concerned with.

Jerry

wrote:
Oh yes, what was I thinking. From the antenna feedpoint, I can run two
equal length pieces of 50 ohm coax to the 1/4 wavelength matching
sections into the T connector - as you just said.

But it will be much more convenient for me to just use an entire run
of 75 Ohm coax, but as I said before, now you have me curious about
the effect on bandwidth - I will have a total of 1.75 wavelengths of
75 Ohm coax per leg.

I just saw the above, after having posted my first reply.

That wouldn't be a smart idea. Compared with either of the solutions
that uses only a single quarter-wavelength for the impedance
transformation, running two mismatched legs to 1.75 wavelengths will
increase the rate of change of phase with frequency by a factor of
1.75/0.25 = 7.

In that case, bandwidth might indeed become a problem, and phase
matching a big problem.


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek