In message , John S
writes
On 7/3/2015 10:17 AM, Ian Jackson wrote:
In message , John S
writes
On 6/29/2015 10:48 AM, Wayne wrote:
As a lead in, I use a 16 ft vertical on 20-10 meters, mounted on a flat
metal roof. The antenna is fed with about 25 feet of RG-8, and there is
a tuner at the transmit end.

While I'm pretty happy with the antenna, I'd like to simplify the
matching.

Thus, the question: what is the purpose of a 1:4 unun on a 43 foot
vertical? ( I assume the "4" side is on the antenna side.)

I'd expect a better coax to antenna match when the antenna feedpoint is
a high Z (example, at 30 meters), but I'd also expect a worse coax to
antenna match when the feedpoint is a low Z (example, at 10 meters).

Is that the way it works, or is there other magic involved?

I think we strayed off the path to answering your original question.

The short answer is that you are correct and there is no magic
involved. A bit longer answer is:

A 43ft vertical will present a feed impedance of 1010 + J 269.2 ohms
at 30 meters. Using a 1:4 transformer at the feed point will reduce
that to 253 + J 67 ohms. That is a bit closer to your 50 ohm line.

A fixed-tuned TX will still need a matcher.


That was not part of the original question(s).



At 10 meters, the antenna will present a 147 + J 133 ohms impedance. A
1:4 transformer will reduce that to 37 + J 33 ohms.

A fixed-tuned TX will probably be reasonably happy with a direct
connection - although maybe even happier with a series capacitor of -J22
ohms.

That was not part of the original question(s).

There are several disclaimers I could include, but I think you
understand that the answers cannot be exact with the info presented.

I hope this helps.

The question is really whether the losses with the 4:1 transformer, plus
those of any matcher at the TX end, exceed those when there is no
transformer (but with higher loss on the coax), plus a matcher. Put
another way, for short feeder lengths, is it better to use the transformer?

That was not the question he asked. Please re-read the OP. I was trying
to address his original question(s) as best as I could. In addition I
also said that there were "several disclaimers I could include" which
may involve your personal concerns. I did not want to muddy the waters.

I think I answered Wayne's question(s), but I will wait to hear from
him to see if that is so.

You have certainly answered "Thus, the question: what is the purpose of
a 1:4 unun on a 43 foot vertical?" (ie to reduce the horrendous
mismatch). However, don't you think there's any virtue in wondering
whether, in the circumstances described (with the relatively short
feeder), it will be any better than a direct connection to the antenna,
and to do all the matching at the TX end? Also, would you use a
transformer if there was hardly any feeder at all, or (in an extreme
case) if the antenna was fed directly from the TX? I'm not advocating
anything - only wondering.
--
Ian