"John S" wrote in message ...
On 7/3/2015 10:37 AM, Wayne wrote:
"John S" wrote in message ...
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.
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.
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.
Thanks John.
Yes, we have strayed from the original question, but I have found the
discussion stimulating.
Indeed! So have I.
Perhaps a new thread should be started to address those subjects.
Please start one if you feel compelled.
If I use EZNEC to model the 43 footer over perfect ground with a 3 inch
diameter radiator, I get impedances in the same ball park as you list.
Ha! I used 1.5 inches. I will re-do.
If I change the "alt SWR Z0" to 200 ohms (presumably what the antenna
would see as a feedline, if a 4:1 unun had 50 ohm coax on the other
side), the SWR plot becomes interesting.
I've never done that. I will explore this set-up.
The plot has SWRs of about 2.5:1 to 5:1 over most of the range, with SWR
getting below 2.5:1 around 29 MHz.
Are we still considering a 10MHz to 30Mhz frequency sweep?
Well, I have been running the SWR across 4 to 30 MHz, but mainly looking at
10 MHz and above.
As for EZNEC and transmission lines, I have never done that, but plan to
when I can. I don't follow how to do it. In the few cases I wanted the
info for a single frequency, I just used a Smith chart.
This thread has given me a lot to consider in improving my whip setup, but
details of the possibilities would run the thread off in the weeds
