> By actual length do you mean the length of the entire conductor, the
> circumference of the loop? That could be a couple inches on VHF
> cavities.
I think some of the comments made in this thread are misleading.
The coupling element (loop, probe, whatever) adds *some* electrical length,
but it's not correct to say that the actual physical length of the element
is what you should use to determine how much to subtract off a true
quarter-wave (or half-wave or whatever) length of cable. How much should
you subtract? Very difficult to say. The amount may even change depending
on the coupling angle (i.e. loop orientation). It is for this reason that
manufacturers, and guys like me out in the field, have an assortment of test
cables that we use to find the ideal length. We also use line stretchers
and "connector savors" (short male to female adapters) to find the ideal
length before cutting the final cable.
> My supply of RG-400 and silver-plated UHF connectors (my old DB cans
> use UHF connectors) is running a little thin and I don't have enough
> to make a lot of trials-and-errors.
You can cut the cable a little long, put a little dab of solder the center
pin, and just slide the ferrule over the braid without crimping. That way
you can re-use the connector until you find the right length, then do the
final crimp on the center and ferrule. You can do the same thing with
old-fashioned PL-259's and UG-176 reducers too, just soldering the center.
> Although we don't have a UHF-connector calibration standard and I
> haven't seen any available even if I could convince the boss they're
> needed, what I did was I made a set of N-to-N cables, which I can
> calibrate, and a set of N-to-PL259 cables the exact same length and
> cable material which I use to set the cans. I'm counting on the
> small differences caused by impedance bumps and change of velocity
> factor in the PL259 won't be critical on VHF. See any big flaws in
> this approach?
Good adapters with Teflon dielectric will have the same velocity factor as
RG400, so the resulting cable length should come out the same.
There will be some trial and error involved unless you have a line
stretcher, there's no easy way around it. One technique, if you don't have
a line stretcher, or a whole lot of test cables cut to incremental lengths,
is to start with a piece of cable that's a little short and then add
high-quality (very important) connector savers or elbows until you find the
right length. If you have a VNA, measure the electrical length (delay) of
the resulting cable (with the adapters) and then cut a new cable of the
right length.
--- Jeff WN3A
