[TowerTalk] Repairing/replacing a damaged feedline Part II

[Jim Brown]

On 3/27/2019 7:59 PM, Bob Shohet, KQ2M wrote:
> 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.
I wouldn't waste expensive hard line on 160M antennas unless the runs 
are very long -- coax loss is pretty low on 160M. For Buryflex, it's 0.2 
dB/100 ft at 1850 kHz. For LMR400, it's 0.17 dB/100 ft.
> 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.

I've been recommending the #31 material for ten years, and this is the 
first I've heard of anything like this. I suspect your experience is the 
result of freeze-thaw cycles with water trapped inside the tape.  
Ferrite materials ARE quite fragile. Here in Nor Cal, I don't have 
freeze-thaw cycles, but if this was happening I'm certain that someone 
would have told me about it!  I used Ty-Wraps to do this.

BUT -- far more important, see my new choke cookbook based on more than 
a year of research and lab work winding and measuring hundreds of 
chokes. k9yc.com/2018Cookbook.pdf   For reasons cited in the text, I'm 
no longer recommending chokes wound with big coax on multiple cores, but 
rather single cores wound with RG400 or pairs of #12 teflon or THHN.

For electrical reasons, #43 material is a poor choice for HF chokes as 
compared to #31. The primary reason is that it's a NiZn material with a 
rather high Q resonance AND impedance specs with a rather broad 
tolerance. This causes impedance curves to vary widely from one core to 
another. From any given box of cores, useful frequency range of the same 
number of turns with the same cable could differ by 50% from one to the 
other!  #31 is MnZn material, exhibits a much broader impedance curves, 
and although it has the same variations in electrical specs, because the 
Q of the resonance is much lower, it's possible to develop designs that 
are much more likely to work for all the cores in a box.

> 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?:

Never assume VF -- it varies with frequency and from one manufacturer 
and cable construction to another. It can be measured in several ways. I 
do it by sweeping a known physical length of line with my Vector Network 
Analyzer with the far end shorted and the far end open, export the data 
as a Touchstone file to AC6LA's ZPlots Excel spreadsheet, which will 
process the data, computing and plotting VF, Zo, and attenuation vs 
frequency.

VF starts out low at very low frequencies and gradually rises until it 
approaches a "sort of" final value in the range of 100 MHz. It is this 
final value that is stated on data sheets. That final (or nominal) value 
ranges from 0.79 to 0.89 for the Andrew, Cablewave, and Decibel hard 
line in my station.

Any lines that you want to use this way should be carefully trimmed to 
length AT THE OPERATING FREQUENCY.  Also, electrical length applies 
precisely at a single frequency. If, for example, you trim for the 
middle of the band, the line will be more or less than a half or full 
wavelength at the band edges.

I'm using CATV hardline to feed monobanders for 15 and 20M. Because 
electrical length varies from top to bottom of the band, I let computer 
software consider all that so that I can optimize the design. I first 
measure the impedance for the antenna in the shack with the VNA, then do 
a TDR to get the electrical length of the line. I then export that to 
SimSmith, which does transmission line calculations. There I subtract 
out the transmission line to get the Z at the feedpoint. Then I add in 
that piece of 50 coax to get to the point where I want to transition to 
75 ohms,  and the 75 ohm coax. In most resonant antennas, there will be 
"sweet spot" along the line where inserting the 75 ohm cable will be 
optimum, and SimSmith can easily model this from your measured data. I 
then tweak the length of the 50 ohm line until I get the best SWR curve 
at the transmitter. It's been five years since I did it, but I think the 
CATV hard line sections are 2 wavelengths on 20M and 3 on 15M.

BTW -- all those measurements can be done with the better vector 
IMPEDANCE analyzers too, and results will be as good as the analyzers 
and the care you have taken in calibrating them. SimSmith and ZPlots are 
freeware, but ZPlots needs REAL Excel.

73, Jim K9YC

>
> 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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