Re: Why did this work (160m antenna)?
Richard, thanks for your patience. I'll try to be more concise, as it is
much simpler than the comments we've exchanged so far.
I tried 2 antennas in different configurations:
=====================================================================
1. Feeding the center conductor of the Inverted L. Coax runs underground 50'
or so, then connects to my radial plate/coax connector. That arrangement
stunk. Band noise S-0, strong signals S-7. Worked about 10 or 15 stations..
Lots of calls unanswered. VERY PREDICTABLE, a poor performer to be sure.
2. Feeding the Inverted L conventionally...same result as 1 above. Perfectly
predictable.
3. Shorting the Coax at the shack end and feeding as a random wire
.....stupid idea...dead short....I didn't even load it, there were no signals
to listen to, OF COURSE. (You have to understand, I was in a hurry and not
thinking very clearly.)
END OF Inverted L Experiment.
===============================================================
Begin CW-80 Experiment: (OCF Dipole with Line Isolator) up 42'. 85' one
side, 51 feet other side.
CW-80: 50' underground coax, then about 45' of vertical coax to the
feedpoint of the CW-80 (OCF)
1. I did not feed it conventionally, as I didn't want to chance heating up
the "Line Isolator" located 22' below the feedpoint of the OCF. In other
words, I didn't just plug the CW-80 coax into the tuner and try to tune it
up on 160. I was afraid this might cause the "Line Isolator" to fry (the one
located 22' below the feedpoint of the CW-80)....that is how the CW-80 is
constructed...it comes with the line isolator, and you attach your coax to
the line isolator. Shack > Coax, abt 95' > Line Isolator > 22' Coax >
Feedpoint.
50' of the 95' from the shack to the "Line Isolator" is underground in a
plastic pipe (along with 3 or 4 other coax cables)
2. Fed the center conductor of the shack end of the coax as a "random wire".
I just pushed the center conductor into the coax connector on the back of
the antenna tuner and made sure the shell was not connected to the tuner.
The worked rather well, as my description earlier details. I'd call this
combo a winner. As I said in my prior post, if I had built an antenna to
work on 160 and got the results I am getting with this option, I would have
concluded that I had a "good" antenna. (For the real estate in use)
3. Shorted the center conductor to the shield and fed that to my tuner
center conductor output as a random wire. (Thus using both the shield and
the center conductor in parallel as a "random wire". This configuration did
not work any better (and perhaps slightly less band noise) than solution 2
above.)
==================================================================
The full layout of the tower and two wire antennas:
Tower is 48'. At 46' or so, I have a 10' metal horizontal cross boom for
pulleys (see below) At 50' I have a 6 element log periodic for 13-30 mhz. at
60' I have a dual band homebrew J-Pole for 2/70cm. So the total vertical
height is about 65', with whatever loading the LP has. The LP only has a 14'
boom.
So, I have a 48' tower with a 10' cross boom at the top section holding a
pulley on each end. One pulley has the CW-80 OCF feedpoint on it with coax
hanging down 5' away and parallel to the tower, to ground level where it
goes into the pipe, underground for about 50' to the shack.
The other pulley on the other side of the cross boom holds up my 80m
inverted L...about 42' vertical and then a sloping wire to complete its
proper length for 80m. (about 25' or so). Its feedpoint is about 6" above
ground level above a radial plate with 33, 60' radials made of #14 THHN
(insulated) wire, stapled to the lawn. The vertical wire is about 5' away
and parallel to the tower (on the opposite side of the tower from the CW-80
OCF.
Hopefully, this clears things up.
The only experiment I'm left with is adding a KW-80 80m trap to the 80m
inverted L and then adding sufficient wire to get resonance on 160. I take
from your prior comments that you don't think this arrangement will work any
better than the "dumb luck antenna" I stumbled into. I'm inclined to agree,
as the 42' vertical section of the Inverted L isn't all that great for
160...but one never knows.
I just don't want to compromise the current performance of the 80m Inverted
L...it is doing a wonderful job on 80m. I worked England on cw and S92RI in
Sao Tome & Principe Is.on SSB, first call. (West Africa). Making repeated
observations comparing the CW-80 (conventional feed) and the Inverted L has
shown the Inverted L receive strength about 2 S-units better on paths beyond
1000 or 1500 miles. On real DX paths, the Inverted L is quite a bit stronger
than the CW-80. Of course, the noise level on the Inverted L is higher than
on the CW-80...all the time. It is rare that I have to listen on the CW-80
and Transmit on the Inverted L....but it has happened. This inverted L
project has been one of my most enjoyable projects in ham radio in years.
Measuring the input Z as I went from 0,2,4,8,16,26,33 radials was a rush, as
was running 2:1 vswr bandwidth changes with each radial increment increase.
The results were downright text book! Falling input Z, decreasing 2:1 vswr
bandwidth as radials were added. Nice predicatable slope.
When Reg gave me his rule of thumb equation for radiation resistance of an
inverted L, that allowed me to begin calculating efficiency based on
feedpoint Z...further fun. And, to compliment Reg, I found two other sources
for the calculation of Rrad of an inverted L and they both agreed with Reg
within an ohm (about 25.4 ohms predicted)...although their formula was
different. Reg must have some sort of magic reference library, or he has
made a bajillion measurements. No matter, his formula worked and was
confirmed by two other sources. So far, I've been able to lower my input Z
to about 29 ohms, so my efficiency (I know... a crude measurement at best,
but better than nothing) is 25.4/29 or 87%. I'll be adding an additional 17
radials when weather and motivation improve, for a total of 50 radials. I'm
not expecting any real improvement in performance, but I have the wire, I
have the plate, I have the ss hardware, and I have the lawn staples. If I
break 90%, I'll be very surprised.
Again, thanks for taking the time to chat about my two projects.
73,
....hasan, N0AN
.
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(rec.radio.shortwave) - Re: Why did this work (160m antenna)?
... Shorting the Coax at the shack end and feeding as a random wire ... I didn't just plug the CW-80 coax into the tuner and try to tune it ... > bandwidth as radials were added. ...
(rec.radio.amateur.antenna) |
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