[TowerTalk] Repairing/replacing a damaged feedline Part II

Thu Mar 28 00:24:37 EDT 2019

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
>> _______________________________________________
>> 
>> 
>> 
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