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1-8 of 8 messages
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homebrew hf vertical
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by HB9TZW on August 8, 2007
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Hi
I'm currently experimenting with a vertical made from old antenna and other aluminium tubes. It is about 25.5 feet high and I have placed a 9:1 unun at the feedpoint. As it is meant to be quickly erected and torn down I have used no radials until now even though I know that they would help. I'm tuning it with an LDG Z-100 tuner.
This setup works for the bands from 40m and up, but not for 80m or even 160m where the antenna cannot be tuned. You will say with this length and without radials it can't work for the lowbands, and probably this is true. But there are commercially sold verticals which are about the same length and of which they say they can be tuned right down to 160m without radials (those cb-looking verticals). Can anybody explain what trick they use?
I was told to use a loading coil in the bottom, but then the result is a lowband-only antenna unless something like a relay switch is used which I do not want.
I would also consider building a completely new or different multiband vertical but I found only very few info about such projects. I.e. does anybody know of any trap-vertical homebrew design (Butternut-like)?
Regards
Martin HB9TZW
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RE: homebrew hf vertical
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by PA3EMY on August 8, 2007
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You could try a loading coil in the middle or higher. Use a capacitor hat to reduce the value of the loading coil. Now you should be able to tune it for the higher bands.
If you don't want to cut your current vertical, just slide some (amidon) toroids over it. You might need a lot of them though. Keep in mind that the can saturate.
good luck
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RE: homebrew hf vertical
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by KB1GMX on August 8, 2007
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I don't know about others but the Butternut HF2V is not a trap vertical. I have one of those.
What they do is use a combination of Tapped inductor
and Capacitor so that at 80M the whole coil is
effective and at 40M the capacitor effectively shorts
out the larger coil. With a bit of effort and trial
I believe this can work for more than two bands by
addional coil taps and capacitors.
In the end once you are above 20M with that 25-26FT
the antenna takes on characteristics of a longer then
1/4wave. That requires a different tuning strategy.
At around 18mhz that antenna is a half wavelength
(very high impedence at feed point) for example.
Experiments here with a 32ft vertical makes it clear
that a tuner at the antenna base rather than at the
radio end of the coax has far more tuning range and
works generally better. If anything it gets past
the cables losses resulting from high SWR out of
the picture.
Allison
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RE: homebrew hf vertical
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by W3LK on August 8, 2007
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<< But there are commercially sold verticals which are about the same length and of which they say they can be tuned right down to 160m without radials (those cb-looking verticals). Can anybody explain what trick they use? >>
The manufarturers lie.
73,
Lon - W3LK
Naugatuck, Connecticut
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RE: homebrew hf vertical
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by W3LK on August 8, 2007
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<< The manufarturers lie. >>
That should be manufacturers, but maybe I was right the first time. :)
73,
Lon - W3LK
Naugatuck, Connecticut
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RE: homebrew hf vertical
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by K5LXP on August 8, 2007
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Well, I wouldn't call it a "lying", I'd call it "selective truth". There's more than one vertical instruction manual out there that recommends a four foot pipe as the sole ground connection. In reality it helps raise the feedpoint Z closer to 50 ohms and widens up the bandwidth, so it appears (to the SWR meter anyway) that the antenna is working great. Using a crappy ground like that could probably help just about any vertical tune wherever you wanted to. So would a 10dB inline attenuator.
Mark K5LXP
Albuquerque, NM
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RE: homebrew hf vertical
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by WB6BYU on August 11, 2007
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What is the return side of the 9 : 1 un-un connected to? Just the shield
of the coax feedline? If so, that is your radial system.
This isn't really such a bad design: with a 9 : 1 transformer feeding
50 ohm coax the SWR is better than 5 : 1 across 20, 17 and 15m.
Well, it would be, anyway, with a good ground system. But since the
impedance is relatively high, the shield of your coax may be an
adequate enough radial system as long as it doesn't give you problems
with RF in the shack, etc.
On the other hand, the antenna is almost a perfect match to 50 ohms
at 28 MHz, with an SWR of 9 : 1 through the transformer. The worst
SWR over 10 to 30 MHz is about 10 : 1. With the wrong coax length
that could present an impedance of 5 ohms to the tuner, which it may
not match very efficiently. Fortunately, with enough coax loss on the
higher frequencies this probably won't be as extreme of an impedance.
Fundamental quarter wave resonance is a bit below 30m, where the
low impedance means that efficiency will be better with a better ground
system. At that point and lower, the impedance of the vertical drops,
and the 9 : 1 un-un is not a good choice for feeding it because it will
transform the impedance to something even lower. You might see how
it tunes with a 1 : 1 (or even a 4 : 1 or 9 : 1 step-DOWN transformer)
on the lower bands.
Again, as the impedance drops the ground losses become more
critical, and a good ground/radial system makes a big difference in
efficiency. A 20 ohm vertical in series with 30 ohms of ground losses
(such as a pipe driven into the ground) gives an excellent SWR with
50 ohm coax, but is only 40% efficient.
One way you can try matching the antenna on the lower bands is to wind
a loading coil in place of the 9 : 1 un-un and tune it to resonance with
a dip meter. Then wind a few turns of hookup wire around the coil to
couple to the coax and check the SWR. The number of turns on the
loading coil sets the resonant frequency and the number of turns on
the link sets the SWR at resonance - there is some interaction but I have
found it fairly easy to adjust.
So back to the orginal question: if you are interested in low SWR then
a good ground may not be important, since earth losses can improve
your match and widen the bandwidth of the antenna. It is not uncommon
for commercial verticals to rely on losses to meet their bandwidth
and SWR specifications - from that point of view a good ground/radial
system doesn't matter. A single ground rod or pipe, or the shield of
your coax as a single radial, may be adequate. To improve efficiency
(especially on the bands where the antenna is less than 3/8 wavelength)
a good ground/radial system makes a big difference, but on 20/17/15m
where the antenna impedance is high and the 9 : 1 transformer gives
an SWR under 5 : 1 the quality of the ground connection is less critical
and the autotuner should have no problems finding a good match. To
work the lower bands you probably need more tuner range, especially
with the 9 : 1 un-un. The base antenna impedance is around 4 - 500j
on 80m and 1 - 1000j on 160m BEFORE the step-down transformer,
and few tuners can match those impedances (let alone doing it
efficiently) unless they are designed for the job and mounted directly at
the antenna feedpoint. In this case it isn't the lack of radials that is the
problem (though that certainly makes performance less predictable)
but the low impedance of the antenna, the step-down transformer,
and the limited matching range of any tuner.
The other way to match such an antenna with a low SWR across a wide
range of frequencies is to put a large 50 to 100 ohm resistor across the end
of the coax, in parallel with the input to the transformer. That certainly
gives you a good SWR, but at some loss of efficiency. (Only 10 to 30dB
or so, depending on the band. But if I can make QRP contacts with one
watt output, running 100 watts to an antenna with 20dB of loss should
be able to do so also.)
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