[Antennas] Question about deedline length and how it might..

[George, W5YR]

Steve, your description is a little unclear: exactly what does "laid flat
to the ground" mean? 

Flat ON the ground or parallel TO the ground. 

On the hunch that it is the latter, I think that maybe two things happened
when you changed feedlines:

1. you went from "some old 9913" which was probably quite lossy to a new
low-loss line.

2. you changed the physical length of the line.

In the first point, that lossy line was acting to shield you from a
considerable impedance mismatch at the far end of the coax, thereby
allowing your tuner to handle the matching situation. As we all know, lossy
line is not desirable, but doesn't mean that something will not work to
some extent.

In the second point, the low-loss line is now letting your tuner see the
load impedance which is now different because of the different line length.
End result is that you formerly had an impedance situation that your tuner
could handle AND I suspect that you had, possibly due to the age of
condition of the coax (which I would bet anything was watersoaked inside
and out) significant loss in the outer braid which tended to suppress
coommon-mode current arising from the direct connection between coax and
ladder line.

Without knowing more, my suggestion would be that you place a current or
choke balun at the junction of the coax and the ladderline and see if that
doesn't help the RFI. I suspect that your tuner is still able to handle the
line input Z so the presence of the balun should have negligible effect
there.

Good luck and let me know how it works.

73/72/oo, George W5YR - the Yellow Rose of Texas         
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe   
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771 I-LINK 11735
Icom IC-756PRO #02121  Kachina 505 DSP  #91900556  Icom IC-765 #02437

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Steve Jones wrote:
> 
> affect RFI. I am using the same antenna that is called a German quad that
> consist of 236 feet of wire laid flat to the ground and feed in a corner
> with a 28 foot matching stub of 450 ohm ladder line to coax and in to my
> shack and connected to a Drake MN2700 tuner. I use to be able to tune this
> antenna on 10 through 20 meters with no problem and no RFI. It is resonant
> across the entire 40/75/80  meter band and I have no RFI when using
> there...even when running full legal limit. However no it is almost useless
> on 10-20 meters because of the RFI. Especially on bands like 10-17 and 15
> meters. The only thing I changed was remove some old 9913 with the hollow
> air dielectric and replace it with some LMR400 and I took about 20' off what
> was about an 80-90' run. Is it possible since nothing else changed...that
> the shorter length of coax is causing this problem???? It is only about 60'
> now. I would be interested in any and all helpful feedback. 73, Steve, NX1G