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@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@contesting.com] On Behalf Of
Bob Shohet, KQ2M
Sent: Wednesday, March 27, 2019 10:00 PM
To: towertalk@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@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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