Paul, I think where the problem with your measurements comes up is that your 41
inches of RG-8 is close to 3/4 wave at 2 meters, not 1/2 wave. Taking a rough
estimate of 19 inches for a quarter wave on 2 meters (the approximate length of
your quarter wave ground plane vertical element) you would have 38 inches as a
half wave. RF traveling in coax does not travel at the velocity of light, it
is slowed by the dialectric material in the coax. The velocity factor of RG-8
is probably near .66, so your half wave of RG-8 coax would be close to 24
inches, not the 41 inches you quote. The extra 17 inches of coax is more than
an additional 1/4 wavelength, considering that the .66 velocity factor is still
in play.
Analyzing your coax, the first half wave (24 inches) looks like a short and the
remaining quarter wave looks like an open circuit - thus giving you your
measured results. (no apreciable change to the load impedance you are feeding)
Odd multiples of that quarter wave gives you the open circuit when the far end
of the coax is shorted.
If you have access to a wide frequency range antenna analyzer, hook your coax
(shorted on the far end) to the analyzer and find the lowest frequency that
shows a minimum resistance and minimum reactance. That is the frequency that
your coax is a half wavelength. This takes in account the connector you would
connect to your Tee and the velocity factor of the coax.
Analysis shows that a shorted stub that is 1/2 wavelength long (electrically,
taking into account the velocity factor) shows a short at the far end.
Analysis shows that a shorted stub that is 1/4 wavelength long (electrically,
taking into account the velocity factor) shows an open circuit at the far end.
I hope this clarifies some of your concerns.
73 - Jim W5ZIT
Paul Plack <[EMAIL PROTECTED]> wrote:
I'm posting this with all due respect to those who disagreed with an earlier
post, and in the hopes of discovering any error I might be perpetuating.
A few weeks ago, a member of the group was asking for help with interference
on the input of a 900-MHz ham repeater from a co-located FM broadcast station.
Among the possible remedies discussed were coaxial stub filters on the
receiver's transmission line. One of the initial proposals was an open,
1/4-wave stub tuned for the FM broadcast frequency, fed on a coaxial
T-connector. This is, indeed, a common method to "trap" a particular frequency.
I set forth that this wouldn't work, as the desired pass frequency was too
near the 9th harmonic of the trap, which means it, too, would be attenuated.
(These traps are VERY wide when fed on a T-connector, and work at all odd
harmonics of the fundamental.) The open 1/4-wave coax trap, sometimes called a
"suck-out trap," is best suited to a case in which the reject frequency is at
least double the desired pass frequency, to avoid attenuation of the operating
frequency itself.
I proposed that better success might be achieved with a shorted, 1/2-wave stub
tuned for the 900 MHz receive frequency, which would be nearly invisible at
the 900 MHz pass frequency, but provide 20+ dB of attenuation at most
frequencies below about 450 MHz. I did this based on experience not only using
such shorted traps, but also after much past experimentation with my Wavetek
sweep generator.
Two subsequent posts took issue with my suggestion. One, from a member
claiming engineering credentials, suggested my trap would appear as a "dead
short" on the operating frequency, and that a shorted quarter-wave trap was
the correct method. No supporting theory was offered.
Another post suggested that a shorted 3/4-wave trap was correct, based on
recollection of an instructor's comment.
I've built and used several of these 1/2-wave traps, but it's been a few
years, and I didn't want to dispute these comments until I'd gone back and
made some actual measurements. I'd drop the matter, but this is too useful a
technique to have it discredited unfairly.
I still have the sweep generator, but not a scope, so I put my MFJ 259B
analyzer, a 50-ohm dummy load, and a 41-inch piece of RG-8M (1/2-wave cut for
2m) on a T connector and look at SWR and impedance.
Here are the resulting measurements of resistance, reactance, and SWR:
146.15 MHz (Shorted 1/2-wave): R=47, X=2, SWR=1.0 (Virtually unchanged from
the dummy load alone)
73.08 MHz (Shorted 1/4-wave): R=23, X=20, SWR=2.4 (Z= about 31 ohms)
73.08 MHz (Open 1/4-wave): R=25, X=26, SWR=2.7 (Z= about 38 ohms)
146.15 MHz (Open 1/2-wave): R=3, X=1, SWR=12.0 (Z= about 3 ohms)
152.8 MHz (Open 1/2-wave): R=2, X=8, SWR=21.1 (Z= about 9 ohms. Note: This was
the SWR peak, higher in frequency than the "shorted" frequency in part because
the braid was folded back, instead of connected to the tip of the center
conductor.)
The readings at 146.15 MHz, coax shorted, were nearly identical with my 2m
ground plane attached in place of the dummy load.
Note that the only arrangement which looks like a "dead short" is the open
1/2-wave stub.
The bandwidth of the shorted 1/2-wave trap on the dummy load was about 12 MHz
for and SWR of 1.2 or less on 2m. Note that this trap would be plenty wide for
use on the antenna side of a duplexer. (The corresponding 900 MHz version
would theoretically be 45+ MHz wide given the same Q.) It also puts the
feedline at DC ground, and serves as a crude high-pass filter below its
fundamental frequency.
It was explained to me by a cavity guru who first showed me this trick that
the reflected energy in the shorted section returns to the T-connector at
near-equal amplitude, and in phase, with the original signal. If the coax was
lossless and the connectors perfect, the impedance of the stub at the pass
frequency would be infinite, making it truly invisible in the system.
In short, (no pun intended,) these measurements look just like what I've seen
for years on my sweep gen. If you can demonstrate where I'm wrong here, based
on actual data, please elaborate. If you're not sure, please cut one yourself
and measure it.
73,
Paul, AE4KR
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