Hi, Gary.
It is not clear from your description exactly how the "sloper" is fed,
where the radials are in relation to the tower, etc.
Quite a few things could be responsible for a change like that.
Not clearing the leaves off the ground over the radials will gradually bury
them and result in a slowly increasing feed R and loss. Clearing the
ground through each leaf fall period with a leaf blower usually clears that
up. You will note that in areas subject to seasonal falling leaves, AM
broadcast antennas have huge nicely mowed lawns over their radials, and
trees have been removed from the radial field. That is not without cause or
entirely for appearances. Buried bare radials will behave differently over
time than insulated radials laid on top of the ground.
The tower and all the wires running up it are HARD-coupled to the sloper.
It is possible to have more RF current in the tower and its cabling than in
the sloper wire. It is possible for the tower and its cabling to be the
major radiator/dissipator of the tower/sloper *system*.
Adding or removing cables on the tower, or changing the terminations of
conductors, if they are not all bonded (or capacitor-RF-bypassed) to tower
at top and bottom, will change the feed measurements. Exactly how the
measurement changes, and how much it changes, depends on environment and
grounding/blocking on those conductors.
Adding/changing/removing yagi's, etc near the top of the tower will change
the "parasitic element" behavior of the tower as seen by the sloper.
Particularly if the ground is poor, simply having a rainy summer can change
the feed Z of the sloper.
Deterioration of the radial conductors can raise the feed Z of the sloper.
Installing a new antenna/feedline or tower within a half-wavelength radius
or so can alter the feed Z of the sloper.
Folks often throw up a wire from a brief description of an antenna, and are
happy if they can work QSO's. Considering how often time and funds for ham
projects are simply not available, I can hardly argue with this, and I
won't, having done the same myself, quite for cause. I will note that
getting on the air on 160 is one thing, and worrying about the
characteristics of a thinly engineered antenna, and that over most of a
decade, is quite another.
The "time-lapse engineering" of the antenna may not have been considered at
erection time, and deterioration may have started the day after the
antenna's erection. It is unusual for a ham to have an academic grade
recording of frequency sweeps and various measurements taken at routine
intervals, along with dated notation of changes made to the antenna farm.
It's not like a pilot's log or aircraft maintenance log which is required
by law, and punishable if omitted. Almost *nobody* has this data, making
explaining over-the-years-changes really difficult.
Allowing leaves to accumulate may be highly preferable from an appearance
or maintenance effort standpoint, but will gradually deteriorate TX antenna
performance. This is an annual problem for those with BOGs in the woods.
After two years, the bottom layer of leaves will have completely decomposed
into soil or proto-soil, effectively burying the radial wires deeper. Six
or seven years is worth half the years in inches further buried. This
softer more permeable, sponge-like new layer will get criss-crossed with
the grateful roots of anything growing in the woods. At the same time this
adds more dielectric material inside the ad-hoc capacitor that is the
sloper wire vs radials. If not increasing loss, the additional dielectric
material has at minimum increased the feedline shunting capacitance of this
ad-hoc capacitor, dropping the resonant frequency.
Given everything above, it's not at all unusual that you would have
experienced changes. Rather more it was guaranteed that you would
experience changes. One of the blessings of elevated radials or an FCP is
the 8 foot separation between constantly changing lossy ground and the
system consisting of bottom of the vertical wire plus counterpoise.
You may want to revisit the choice of 160 antenna.
73 and good luck,
Guy K2AV
On Sat, Aug 18, 2018 at 7:17 PM Gary Smith <Gary@ka1j.com> wrote:
> Folks,
>
> I'm starting to get ready for the upcoming
> winter frolics on 160. The 160 antenna is
> a sloper and I have somewhere around 50 or
> so 130' radials pretty much buried under
> 6-7 years of leaves. When I went to the
> remote coax switch & checked the readings
> on the sloper with 10' of coax, I found
> some readings with the old MFJ-259B that
> concerns me.
>
> I thought I'd cut the Sloper at 129' long
> for best SWR at 1.825 but I'm now reading
> the lowest SWR at 1.737 MHz and the
> impedance read 85. Going back to the shack
> I read the antenna (with 360' of 7/8 50
> ohm commscope now in-between), and the
> resistance drops to 55 but the SWR still
> reads lowest around 1.737
>
> It appears I need to shorten my antenna,
> I'll have to work out the proper length
> again but my concern is why at the feed
> point would I see 85 for the resistance at
> resonance.
>
> Suggestions on what might I give a look to
> when I go back out tomorrow?
>
> Thanks & 73,
>
> Gary
> KA1J
>
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