You don't state which manufacturer's hardware you have or what yagi design
they are (or if they're yagis for that matter).
Assuming they have some sort of adjustable matching network on each, such as
a gamma match, very common on VHF/UHF antennas, what I would do is readjust
the matching device and run the 75 ohm cable terminated in its
characteristic impedance. The last time I played with VHF yagis was years
ago, but at that time they were capable of being adjusted to match 75 ohm
feedlines, since I did exactly that for similar reasons.
I wish you had included more info on what these antennas are and how you
intend to use them. I'm tempted to say, as do others, to just run the 75
ohm cable with the theoretical 1.5:1 SWR and let it be. However, to that
1.5:1 theory you also end up seeing the results of other impedance bumps
caused by your connectors. With hardline I've seen some interesting homebrew
ones that, on UHF, would cause me to change the discussion from one
involving feedline impedance to one talking about the impact of connector
design at UHF. So, while your "floor" SWR is 1.5:1, your "ceiling" SWR, and
therefore overall feedline loss, is unknown. Short of measuring the loss in
the resulting cable, from Tx to ant, you're in the dark.
Finally, do you have antenna mounted preamps or are you accepting the noise
figure degradation introduced by the line losses? Even if the loss
introduced by a 1.5:1 SWR on UHF ends up being only a dB, that is
significant in terms of noise figure for SOME types of operation.
The ARRL antenna book 1997 version, page 24-16, leads me to believe you've
OVER estimated the loss in the 9913. The book says it's 3db at 450MHz, not
6 dB as your state. For 1/2" 75ohm hardline (HL), it's 2.5db at 450MHz.
Not a helluva lot of difference, when you consider the headaches of running
the hardline, getting connectors that do the job at UHF, etc. Throw in the
SWR/losses caused by running the hardline unmatched and/or using
questionable connectors, and you're probably at a wash or worse. If you
have 3/4" HL, you gain 1 dB over the 1/2" HL, but there's still the unknown
with the connectors and matching.
Strictly by the numbers, and assuming your 9913 is in good shape, I'd
continue to use the 9913. I'd measure the loss of the 9913 at 450MHz and
compare to spec. If it's only 3 db and if the 3 dB loss is bothersome,
improve the antenna (for TX) or add some antenna-mounted preamps (for RX) or
both. Seems like the feedline is not the best bang for the buck or the ergs
of personal energy involved, IMHO.
| ----- Original Message -----
| From: "Michael Foerster" <firstname.lastname@example.org>
| To: <email@example.com>
| Sent: Saturday, March 31, 2001 7:02 AM
| Subject: [TowerTalk] Matching 75 ohm hardline for 3 bands
| > Is there a solution for matching 75 ohm hard-line to a triband antenna?
| > I have a 6 meter, 2 meter and 440 antenna on my tower with 100
| ft of 9913
| > feeding it. At 440, it's probably about 6 dB loss. I would like to
| > replace the bulk of the line with 75 ohm hard-line. I know that you can
| > match the impedance with tuned matching stubs, but that's
| difficult to do
| > with 3 different bands at once...
| > Any suggestions?
| > Mike
| > W0IH