[TowerTalk] Repairing/replacing a damaged feedline Part II

[Bob Shohet, KQ2M]

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