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1-5 of 5 messages
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FT-817 and LDG Z-100 Dipole Recommendations?
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by KC0KEC on May 17, 2009
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Hello,
I have an FT-817 and an LDG z-100 tuner. Although I have a nice homebrew portable multiband dipole (uses alligator clips to change bands) I would like to get a dipole HIGH up in the trees permanently for base operation with my FT-817. Later, I'll get a 100w radio for the base, but for now I need to use the FT-817.
With this combination of equipment, what dipole length would you recommend for 40/20/10 meter operation with this tuner? Should I go with the longest overall length (40 meters: 65.34 feet based on 7.0 to 7.3 Mhz) and let the tuner deal with it? The LDG is designed for coax, so I'll be feeding the antenna with coax.
Would appreciate any suggestions.
Thanks,
KC0KEC
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RE: FT-817 and LDG Z-100 Dipole Recommendations?
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by WB6BYU on May 17, 2009
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If you are feeding the dipole with coax you have to keep
the SWR down to a reasonable level. Just because the
tuner can match it doesn't mean that you don't have high
losses.
Here is an example: Say we have a 20m dipole fed with
100 feet of RG-213 coax. The feedline loss is about three-
quarters of a dB, which is quite acceptable.
If we try loading the same antenna on 30m the SWR is
over 100 : 1 and the loss is over 9dB. About 12% of your
power makes it to the antenna. Actually you'd only
measure an SWR at the rig of around 13 : 1 due to the
high losses.
Going up in frequency, on 17m the SWR is about 30 : 1 at
the antenna. Now about 3/4 of your power is lost in
the coax and 1/4 of it actually gets radiated.
The situation gets worse as you move to bands further
from the resonant frequency, so a single dipole wire
with coax feed is not likely to work well on more than
one band (or perhaps two, such as a 40m dipole used on
15m.)
There are ways around this, however. You can use a trap
antenna, a G5RV, or some other design that gives low
SWR on the bands of interest. If you can keep the SWR
under 5 : 1 at the antenna the losses shouldn't be
more than 2dB on 15m.
The other approach is to use a more efficient feedline.
Most types of parallel conductor line (twinlead, ladder
line, etc.) will be more efficient at high SWR than coax.
In that case you can bring the feedline down to the
shack and run it through a balun to convert it to coax
right at the back of the tuner. That minimizes the
feedline loss. If you are going to use this approach
then I'd suggest your dipole be at least a half wave
on the lowest band you want to use (so round 125' on
80m) and it should work with good efficiency on all of
the higher bands.
Or you can find lower loss coax cable: some CATV trunk
hardline might keep your losses down to 6dB on most
bands - that is 25% efficient.
VK1OD's transmission line calculator is a handy tool
to use to evaluate the losses in various feedlines:
http://www.vk1od.net/calc/tl/tllc.php
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RE: FT-817 and LDG Z-100 Dipole Recommendations?
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by KB1QQA on May 18, 2009
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I built a 20 meter dipole using some 14AVG wire purchased at Home Depot for $6, and a piece of spare coax. Trimmed the dipole to an SWR of 1:1 at 14.250.
I'm running an FT-817nd powered by a 12V 7Ah SLA battery. The dipole is mounted in a attic of a two story home(it is about 25 feet above ground). From CT, with 5 watts, I'm making contacts into Texas, Florida, and much of the mid-west, as well as Costa Rica and Spain.
If you want a multi-band the Par EF10/20/40 has great reviews. I bought one, but did not get a chance to tune and use it yet. Best of all no tuner required.
Since you have a tuner, I'd build a G5RV jr. and give it a try. Some wire and twin lead, and you're ready to go.
Good Luck and 73
John/KB1QQA
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RE: FT-817 and LDG Z-100 Dipole Recommendations?
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by WB2WIK on May 18, 2009
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Dale's right on all counts. You can't coax-feed a multiband HF dipole unless it's a 40m dipole you'll also use on 15m. All the other "combinations" don't work well for a coax fed antenna.
But you could homebrew a parallel dipole easily, or if that's too daunting, use a commercial parallel dipole like the Alpha-Delta model DX-CC, which works *very* well and covers 80-40-20-15-10 meters all in one antenna. It matches very well even without a tuner on 40-20-15-10 meters, and at one point in 80m. You can extend 80m coverage to "the whole band" using a good tuner.
WB2WIK/6
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RE: FT-817 and LDG Z-100 Dipole Recommendations?
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by KC0KEC on May 19, 2009
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Thanks all for your help!
I am very intrigued by the G5RV. I believe I'll give that one a try
73's
KC0KEC
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