I would expect the outer is a Teflon or Teflon-like material, with
probably low loss and fairly low relative dielectric constant, but it's
still going to require that you shorten that lower section quite a bit
to get the quarter-wave stub to reflect a high impedance. In addition,
the dielectric between the outer of the coax and the inner of the
sleeve is pretty thin, and the impedance of the resulting coaxial
arrangement is pretty low. That means that it won't ever reflect a
very high impedance. You can use a tapered sleeve that bells out at
the bottom, to good effect. That will also lower the feedpoint
impedance and match better to 50 ohms. Think: ground plane with
drooping radials. But you can also wind the coax just below the
antenna into a small coil (I'd make the axis of the coil coincide with
the axis of the antenna) that's self-resonant near your operating
frequency, and it will very effectively choke off antenna current
(current on the outside of the coax) at that point. Put one such coil
an inch or so below the bottom of the sleeve, and another about a
quarter wave further down the line.

SWR meter: Get RFSim99--do a Google search for it. Build a coupler,
per toolsdesigncoupler. I'd suggest a microstrip version, if you
can make a little PC board reasonably accurately. Design it for 50
ohms. Terminate each coupled port in 50 ohms (e.g. 49.9 ohm 0805 SMT
part). Using vanishingly short leads, connect a simple diode detector
to each of those two loads. Use a calibrated attenuator to calibrate
at least the relative response of those detectors. Use those two
outputs to calculate SWR. You can read the diode detector outputs with
a DVM that has good resolution (10uV sensitivity preferred; 1uV is even
better). I'd recommend about a 20dB coupler for the power level you're
using, though even a 30dB coupler would work. Try terminating the
through line in a 49.9 ohm (or a parallel pair of 100 ohm) 0805 parts
to check that you see essentially no reflected, and try a 100 ohm load
and a 25 ohm load to check that you get the expected reflected. -- To
have the coupling right according to the RFSim99 directions, it needs
to be 1/4 wave long, but it's a pretty broad peak. Coupling drops to
zero at 1/2 wave, and at DC. So you could make one for 900MHz, and it
would work OK at 450MHz, you'd just get lower coupling. The ratio for
SWR would still be OK. The 450MHz version on FR4 board
(fiberglass-epoxy) would be roughly four inches long, if my mental
arithmetic is right, and half that for 900MHz.

Cheers,
Tom


David wrote:
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