Hi,
I am in the process of replacing the 160 meter feedline – initially with
Buryflex and then in short order with hardline. That will be the best
protection against critters and other sources of damage.
But when I was looking at the feedline today I noticed that the 31 Material
used in the ferrite cores for the 160 feedline choke had literally
disintegrated – tiny crumbs held in shape together with electricians tape that
felt apart when I took the tape off. All 11 cores were similarly
disintegrated. I have been using cores with 43 material for years with NO
disintegration. Is the 31 Material normally this fragile or did I get a bad
batch? Although it has been out on the feedline for the past 6 years, the
electricians tape prevents exposure and keeps the cores from moving around in
the wind. It was –20F (without the wind chill) 3 Winters ago – is that cold
enough to cause the cores to break? Very odd.
Now a question... as AD3F pointed out and from my understanding, at even
multiples of 1/2 wavelength, the impedance of the antenna will be unaffected
whether I am using 50 ohm or 75 ohm coaxial cable or hardline. So... carrying
this concept forward for a 160 meter feedline (resonating at 1.825 Mhz), if my
goal is to make a 1 wavelength feedline electrically to feed an Inverted L, and
using both RG11 and hardline, I posit the following for you to please check and
verify my math.
Assuming that the Vf of RG11 (75 ohms) is 0.78 and the Vf of the hardline (75
ohms) of is 0.84, then would the following work?:
For RG11 984/1.825 x .78 = 420.56’ for 1 electrical wavelength. If I want
0.05 wavelengths (just enough to bring the feedline down to the ground where it
can “mate” with the hardline) then I want 21.03’ feet. Then, subtracting the
0.05 wavelengths from 1.0 wavelengths total would leave me with 0.95
wavelengths for hardline.
For hardline 984/1.825 x .84 = 452.91’ x .95 = 430.26’
So to equal 1.0 electrical wavelength I would need 21.03’ RG11 + 430.26’ of
hardline, and if this is correct then the antenna will be properly matched
without the need for a matching section and the swr should be 1.4:1 .
Is that correct?
If not, what is not accurate?
Incidentally, for the person that asked, Buryflex is RG8U / 9914F – that is
what the sticker says on my roll.
Tnx & 73
Bob, KQ2M
From: Bob Shohet, KQ2M
Sent: Sunday, March 24, 2019 9:02 PM
To: yccc@groups.io
Subject: [yccc] Repairing/replacing a damaged feedline
Hi,
Today I noticed that my 160 feedline which is a ~ 325’ feet long run of
Buryflex, had lots of critter teeth marks in two main areas. Clearly the coax
in these areas is damaged beyond repair.
Starting at the feedpoint of the 160 Inverted L, the first 25’ runs downhill
and is pristine with no critter teeth holes. Then they start to appear and run
off and on for the next 100’ or so. The is enormous damage at the 100’ mark –
about 75’ from where they start appearing, and from start to finish, the length
of critter damaged cable is ~ 100’. This entire area is on flat ground.
My understanding is that foam tends to wick up moisture but for how far on flat
ground? Can it wick 25’? 50’? 100’? The entire length of the buryflex? I
don’t want to wind up using and damaged or deteriorated cable but I don’t want
to needlessly waste 150’ – 200’ of potentially good coax
The coax run is ~ 6 years old so aside from the critter damage and any possibly
wicked up moisture, the rest of the cable still has many potential years of
life left.
So my question is – should I just replace the entire 325’ Buryflex feedline?
Or does it make sense to replace the the 100’ damaged area plus an additional
“safety” area of ~ 50’ for a total of 150’?
Tnx & 73
Bob KQ2M
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