[TowerTalk] Repairing/replacing a damaged feedline Part II

[Jim W7RY]

Agreed

Sent from my wireless communicator

On Wed, Mar 27, 2019, 11:28 PM Stan Stockton <wa5rtg at gmail.com> wrote:

> Negligible at 1.8 MHz.
>
> Stan, K5GO
>
> > On Mar 27, 2019, at 10:38 PM, Gary Schafer <garyschafer at largeriver.net>
> wrote:
> >
> > 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 at contesting.com] On Behalf Of
> >> Bob Shohet, KQ2M
> >> Sent: Wednesday, March 27, 2019 10:00 PM
> >> To: towertalk at 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 at 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
> >> _______________________________________________
> >>
> >>
> >>
> >> _______________________________________________
> >> TowerTalk mailing list
> >> TowerTalk at contesting.com
> >> http://lists.contesting.com/mailman/listinfo/towertalk
> >
> > _______________________________________________
> >
> >
> >
> > _______________________________________________
> > TowerTalk mailing list
> > TowerTalk at contesting.com
> > http://lists.contesting.com/mailman/listinfo/towertalk
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>