Tom,

Thanks for the information. The inner coax is the smaller RG174 coax.
The sleeve is made of earth braid pulled from RG58 cable. The dielectric
between the sleeve and inner cable is therefore the outer sheath of the
RG174 cable (Not sure what this is, the RG174 I have is Teflon inner
dielectric and stranded conductor. The utter sheath is a very strong
heat resistant material - I therefore have no ideal of the dielectric
constant to calculate Vp for correct electrical length). If I use a
copper tube and strip off the sheath from the inner coax, I can
calculate correct length as it will have an air dielectric.

Do you know where everyone is getting the dielectric constants for
various materials ? I noted people using small metal tubes as sleeves
and quoting these magic numbers even for copper tubes of certain diameters.

From my discussion with Telonic, they say the Rho_Tector was designed
as an in-house tool for measuring inputs of amps and filters. They
suggest SWR meter would probably be best for antenna adjustments.
Do you happen to know where I might find details for a low power SWR
meter for 915 MHz ? I need one that will operate with only 20mW applied
power. The only SWR meter I have has min. FSD of 3W

Thanks

Regards

David

K7ITM wrote:
Hi David,

You wrote, "The decoupling is via a 1/4 wave sleeve that provides high
impedance for
RF returning along outer coax and also as the second 1/2 of the
dipole. "

EXACTLY how is this built? The details of construction make a BIG
difference in performance! (There's a lot of BAD info about it out
there...)

It's not a bad idea to ALSO put some additional decoupling further down
the feedline.

If your spectrum analyzer/field strength meter is far enough away from
the antenna you are testing, then it should provide a reasonable
indication of relative antenna radiation performance. The SWR
indication, if properly calibrated and given that you are apparently
exciting the antenna with a source whose output impedance matches your
feedline, should also be a good indication of power actually absorbed
by the antenna. That is, lowest SWR represents maximum power absorbed
by the antenna. Presumably that power is being radiated as RF, mostly,
and not dissipated as heat. But where the RF radiation goes depends on
the pattern of currents excited on the conductors that compose the
antenna, and nearby conductors as well (such as the feedline). What
you probably want is standard resonant half-wave dipole currents on
your vertical dipole, and no (very little) antenna current on the
feedline and on support structures. By the way, whether the antenna is
resonant or not is of little real importance, so long as you can
efficiently feed power to it and the antenna currents are in the right
places and not the wrong places. But it happens that with your
antenna, if things are working properly (properly decoupled feedline,
etc), you probably will see lowest SWR at half-wave resonance. If you
have no other matching going on, the lowest SWR will probably be about
1.5:1 with 50 ohm feedline. You could add parts to get a better match
if you wished.

And as you can probably tell from all that, I'm suspecting that your
decoupling sleeve, with associated dielectrics in that area, probably
isn't doing a very good job...

Also...Joe noted that your coax feedline may well be a length that
accounts for the SWR peaks and valleys. (I think it may be about twice
as long as Joe wrote...but same idea.) Do you see the peaks and
valleys when you terminate the line in the precision 2:1 load? If you
do NOT, then it's a further indication that the feedline has antenna
currents on it, because the flat 2:1 is an indication that your
transmission line is matched to the calibration impedance of the SWR
bridge, and if that's the case, the SWR bridge should be giving at
reasonably accurate estimate of the actual line SWR. If you DO see the
SWR ripples vs frequency with just the precision load, either the load
isn't "flat" or the line is not the same impedance as the SWR bridge is
calibrated to, and the differing impedances is by far the most probable
explanation if the line length is right.

Cheers,
Tom