On Fri, 29 Sep 2006 08:10:01 -0400, jawod wrote:

jawod wrote:
I spent the day lowering the folded dipole that I BOUGHT. I was unable
until recently to find how bad the SWR was on most bands. The only
reasonable SWR was on 17 meters.

So, I replaced it with a homemade G5RV, measured appropiately for each
leg of the dipole (14AWG) and for the 300 ohm window line to R/G8U.

I now find reasonable SWR on most bands (WARC, not so much). Here's the
deal: min SWR comes in right at 3.5 MHz, 6.75, 13.43, 18.07 and 29.06.
It appears that I made the classic newbie mistake...antenna too short.

Now, since I am using an ATU, I think this should be close enough for
acceptable efficiency (at least for 80, 40, 20, 17 and 10 meters).

What do you think?

John
AB8O

PS, When I get the time, I'll try EZNEC but for now, I just want to work
what I hear for a change (!)
Update:

Well, yes, the antenna was too long (not too short). I decided to lower
the antenna and shorten each leg by 18 inches. Now it's back up about
45 feet, sloping to about 25 feet. (No change from before.)

Now, SWR dips are at 3.610, 6.710 and 13.580 MHz.

These are not what was expected. Oddly, on 40M SWR dip went further
away from the band edge (as if dipole was lengthened).
I did not change the 300 ohm window line (at 31 feet).

From what I've read here and elsewhere, I should note 80 and 20M
performance as primary considerations.

I'm a little nervous about shortening it further.


I have explained to you several times that the length of the radiator
and the open wire section act together to influence the impedance
presented at the coax to open wire junction.

I asked if you had measured both elements and if you had considered
the velocity factor (for your actual line) in those measurements, but
you did not reply, and you have not reported the length of the open
wire line. The surest way is to measure the velocity factor, calculate
the correct length and accurately cut to that length.

The common G5RV has dipole's second series resonance at 14.2 and the
open wire line an electrical half wave at 14.2, which will present a
resistive load of somewhere 80 to 90 ohms at the coax to openwire
junction, and you will observe a VSWR minimum on the coax at that
frequency (even at the tx end of the coax line). (Of course if you do
something silly like use a 50 ohm VSWR meter in a 75 ohms coax line,
you aren't making valid VSWR observations.)

Remember the rule: measure twice, cut once.

It can't be all that hard, can it.

Owen

PS: I noted your throw away comment that time is of the essence, the
wet season is coming. Have you noted the recent comments on
performance of wet ladder line. Depending on the construction, your
300 ohm line might be subject to similar degradation, and your
"optimised" configuration might not be optimised when the feedline is
wet.
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