The only thing that a half wave or multiple of does is repeat the impedance
seen at one end, at the other end. It does not match anything.
And it is only true at one frequency.
There will still be SWR on the line from the mismatch of the coax impedance
and the antenna impedance but the antenna impedance will be seen at the
other end of the line regardless of what the coax impedance is.
But because there is still SWR on the line from the line to antenna
mismatch, there will be additional loss in the line due to the SWR.
73
Gary K4FMX
> -----Original Message-----
> From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
> Bob Shohet, KQ2M
> Sent: Wednesday, March 27, 2019 10:00 PM
> To: towertalk@contesting.com
> Subject: [TowerTalk] Repairing/replacing a damaged feedline Part II
>
> 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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