On Thu, 31 Jan 2008 04:17:34 -0500, K8RI wrote:
>like something more flexible like RG8-X, but RG-8X has relatively poor
>shielding.
Hang on a minute. RG-numbers for coax are VERY generic -- there are at least
a dozen cables that fit the RG8X designation, and they are different. RG8X
is simple an intermediate SIZE between RG58 and RG8. LMR240 is simply
another RG8X, and, IMO, a very nice one. The Wireman sells at least two
RG8Xs, and they are different. I've used both his CQ118 and a Belden RG59
designed for transmitting on dipoles running 1kW and never had a problem,
even when on a part of the band where the VSWR was pretty high.
>I'll be using crimp connectors and flooded heat shrink for good mechanical,
>waterproof connections. I'm just tryin to get away from the weight of the
>RG8 and LMR 400 cables for these antennas.
I've got high dipoles that require about 150 ft of coax. When I was deciding
how to feed them, I spent a lot of time with NEC studying the antenna
impedances, and TLW computing how much loss various feedlines would
contribute. Taking off-resonance operation into account, the difference
between RG8 (or RG11) and RG8X (or RG59) can be 1.5-2 dB on 160-40M, even
more with higher VSWRs. My 80/40 fans come pretty close to a match on 30M
and 17M, so the fact that I've got RG11 on them rather than RG59 is probably
worth 2-3 dB on those bands. When I had less robust supports for them, I
used RG59. My high 20/15/10 fan dipoles look like 50 ohms, so I'm feeding
them with RG8.
If we want to make intelligent decisions about which coax to use, we need to
stop thinking about RG numbers and start paying attention the construction
and the loss specs of these cables. Belden makes several dozen RG59s, and
they are very different from each other. They also make nearly a dozen RG6s,
also very different from each other. Some are designed for transmitting,
some only for receiving. Some have thin foil shields, some foil with thin
braid, some foil plus heavy braid. Likewise, center conductors vary both in
size and metal, and the dielectrics and outer jackets are different.
We also need to re-think our reliance on 50 ohm cables for 75 ohm antennas.
Thanks to interaction with the earth, a LOW dipole is usually closer to 50
ohms than 75 ohms, but a HIGH dipole is likely to be a much better match to
75 ohm cable. The LOSS in that cable depends on the match between the coax
and the antenna, NOT the match between the transmitter and the antenna, AND
we can use a tuner to match the feedline to the transmitter.
Also, there's a great piece by AI1H in the ARRL 6th Antenna Compendium
showing that below 1 GHZ, loss in coax is primarily copper loss, not
dielectric loss, so coax with a robust low resistance shield will have less
loss than one with a thin high resistance shield, especially on the lower HF
bands. Further, the SHIELDING effectiveness is directly related to the
RESISTANCE of the shield at the frequency of interest.
There is some excellent stuff in these Antenna Compendiums. I've learned a
lot from studying them.
73,
Jim Brown K9YC
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