On 10/1/2015 1:56 PM, Ian Jackson wrote:
In message , Brian Howie
writes
In message , John S
writes
On 10/1/2015 3:29 AM, Brian Howie wrote:
In message , Wayne
writes


"Ian Jackson" wrote in message
...

In message , rickman
writes
On 9/30/2015 12:57 PM, John S wrote:
On 9/30/2015 10:12 AM, Ian Jackson wrote:
In message , Jeff
writes
On 29/09/2015 14:31, Jerry Stuckle wrote:
On 9/29/2015 4:40 AM, Jeff wrote:


So let's get back to the original question. Was it ever really
answered? I think it was made slightly more complicated by the fact
that the antenna feedpoint impedance was not purely resistive, but
was actually around 20-j130 (at 14 MHz),

Was there any advantage in having the coax paralleled (both for 20
ohms resistive, and for 20-j130)?

A quick model.

A vertical antenna about 4.2m long with a wire radius of 0.5mm
approximates to what you have . The devil is in the j130 If you use a
1.5uH series L the SWR for the parallel 50ohm line is 1.19: 1 and
not a
bad match over the band

Expanding on the original question.... Antenna feedpoint
approximately 20-j130
The ATU drives the antenna through about 15 feet of coax.
Assuming that the ATU provides a +j130 conjugate match, does that
leave the coax with a SWR of 50/20= 2.5:1?
If so, then I will not bother with considering 2 parallel coax.

No the line SWR is still about 36:1. If you run a bit of poke, you
might
melt the coax.

If you want a single 50 ohm feed, extend the antenna to 5.7m ( ~50+
j87) and use a series C ~130pf to take out the the +j87. You have to
do this at the antenna end.

It goes without saying you need a good ground.

Brian

What do you think of this while leaving his antenna unchanged?

2.3uH
___
'-----o-----UUU---------------
.-. | ^
| | | |
20 | | C| '
'-' C| 4uH
| C| 50 ohms
| |
--- | .
-130--- | |
| | v
'-----o------------------------
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


Yes that works too. Lots of ways to do it.

I'm taking the opportunity to refresh my admittedly rudimentary skills
with the Smith Chart - and in particular, single- and double-stub
matching (little used for over 50 years!!).

However, in Wayne's situation, the length of the coax is only 15'. On
14MHz, that's just over a quarterwave (taking the velocity factor into
account). But even with a horrendous SWR, how much loss does this length
of 213 coax have? It might be a lot more convenient to do all the
matching in the warmth and comfort of the shack.

He is already doing that with his ATU and is pretty happy with it. And
he has indicated that he doesn't want to add matching components up at
the feed point. Brian and I were just bouncing ideas back and forth.

I have done exactly this with a 130' inverted-L Marconi-type antenna,
fed at the far end directly with around 100' of old (early 1960s)
semi-airspaced TV trunk cable (with a good ground there). It worked fine
on 160-80-40m (the bands I was interested in working), but it loaded up
fine up to 10m - and as it seemed lively enough on receive, I'm sure it
would have put out a reasonable signal. Although I eventually treated
myself to a remote automatic ATU, I'm not convinced the system works any
better than it did with the direct coax connection.