[TowerTalk] Repairing/replacing a damaged feedline Part II

Thu Mar 28 00:42:23 EDT 2019

I've had type 31 ferrites on a bunch of antennas over the years and 
never had a crack that was not caused by physical damage (meaning I 
dropped them or hit them somehow).  So if you have cracked ferrites then 
something is wrong - they definitely do not respond well to shear forces 
and are very easy to crack by impact.

73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com

On 3/27/19 11:24 PM, Stan Stockton 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
>>> _______________________________________________
>>>
>>>
>>>
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