Hi Steve,
5300 pF is way large. It indicates *something *else is going on.
"Something" needs to be determined.
One thing for sure, after nearly a decade of correspondence with this as a
frequent subject, there is no one single "silver bullet" to fix this in all
cases. It is complicated and with several distinctly different causes, each
one by itself capable of causing the symptoms you report. Only *one *of the
several causes is* not* also causing significant RF loss that minimizes
your radiated TX power.
Worse, quite often two or three of them are in effect at the same time. And
excellent reports and suggestions by those trying to help out seem like an
argument about which solution is "the one". In fact, all of the respondents
may be making an excellent suggestion about *one *of the* several ways* the
problem reported above can be caused.
Very unfortunately, one may have to fix all two or three or four to get the
antenna acting with an ideal modeled result.
*So, apologies for the length*, but this one possibly takes a silver
bullet, plus a gold bullet, plus a platinum bullet, plus a depleted uranium
bullet to solve all the possibles responsible for this report. So on to the
stuff....
The *large* capacitor needed means that the amount of inductive X being
tuned out *is getting small.* If what you did was lengthen the Inv L
horizontal to get 50Ω R and then use a series capacitor to tune out the
inductive X, the method has a blind spot where the* inductive reactance at
R=50Ω is so low that the cap has to be huge. *
This is usually caused with an Inv L because there is* a lot of **fixed RF
loss R somewhere* added to the L's natural 20-35Ω radiation R at X=0Ω. The
Inv L is decidedly not a natural 50Ω antenna. X=0Ω should not be close to
R=50Ω
If you take away the added loss, the now needed extra length to get R=50Ω
all from radiation resistance (lengthening the horizontal) is substantial.
The longer length has a lot larger inductive X to tune out. This *reduces *the
size of the cap needed. *Smaller *pF value caps produce the *larger
*capacitive reactance
to cancel out that larger inductive X.
And of course it could be that something in the environment is giving you
an "X push" one way or the other.
So exactly *what *is the added RF loss, or X push, and *what *is causing
it?
To start, you have an *undefined* tower involvement which is capable of
producing a very large RF loss addition to that tower-supported 1/4
wave-ish L, *and at the same time *also capable of producing a very large X
push in a capacitive *or *inductive direction.
Anything in the k2av.com "Loss List"
<https://k2av.com/K2AV_LossList.html> could
be adding ohms to the feedpoint R.
One of the splendidly frustrating things about 160 meter antennas is,
unless we can put up a 160 dipole at 250', we probably need to go vertical
oriented.
We can't do problem-solving on vertical-antennas-for-160 accurately or
effectively without considering several overwhelming factors on 160: Loss,
ground effects, and a monstrous wavelength which multiplies miscellaneous
conductor involvement.
These can't be reliably determined or solved by tuning for SWR. You're only
trying to match* (antenna + problems)* to 50Ω.
The inverted L with the bend supported by the tower, and fed at the
radials, is really a transformer in disguise. *The L is one winding of the
"transformer". The tower and each coax shield and control conductor on the
tower are separate windings in this multi-winding ad-hoc "transformer".*
Somewhere in my stuff I have a NEC 4 model of an L supported at the bend by
a tower that has more induced current in the tower than there is in the L.
This transformer situation has an *effective *turns ratio that keeps the
tower with lower voltage and higher current. This higher current is then
driven into and dissipated in the ground.
If your tower cabling has:
All its shields and unused conductors in the cable grounded to the tower at
the base...
And all active control conductors bypassed to the tower at the base…
Then Tree's suggestion to detune the tower works to its maximum
effectiveness.
But tower detuning has to be done well. Otherwise the induction to the
tower will still drive a lot of current into the ground. The induced RF
current in ungrounded or unbypassed coax shields and control conductors
will be driven into ground via capacity effect all along their lengths
laying on or buried in the ground between the tower and the shack This loss
adds to the R of the L feed through the above transformer effect.
A second issue is whether the radial's center is solidly connected to the
tower's cable grounding point. That will substantially reduce the dirt's R
that the induction is forcing RF into.
One case I was involved in violently changed the feed Z when the tower base
was bonded to the radial feed. The performance picked up substantially and
he only needed a 5:4 turns ratio transformer to get close to 50 ohms after
pruning the L for X=0. Until then, the true radiation resistance was in
series to an apparent effective loss resistance caused by the tower base
acting as a simple ground rod at RF frequencies. Lossy, lossy, lossy. And
an inductive X was in the blind spot since his big vacuum cap was only 2000
pF.
The philosophical basis of all the preceding is take care of all loss
issues and only then do the matching. After loss remediation and after
unwanted-coupling remediation, 160m antenna feedpoint matching is usually
way simpler than before. This is because the unwanted stuff is highly
variable, unpredictable. What bedeviled one's buddy ham across town often
not the same as what's digging at you. His experience may not help you at
all, even though he was in fact successful, and telling the absolute truth.
You got good radials down. 50 times 120 should be a splendid almost zero
ohms RF ground for the tower. A very low RF R from any counterpole (defined
on k2av.com) <https://k2av.com/K2AV_DesignInvertedL.html?#cpoledef> is a
huge start on effective 160m TX.
As Tree said, detune the tower. Make sure all cabling is grounded or
bypassed at tower base, and bond radials to the tower.
Compared to feed Z with detuning connected, disconnecting the detuning
should significantly change feed Z. If it does you need to keep the
detuning.
Also see if radials bonded to the tower versus unbonded makes a big
difference. If it makes a difference, then you need to keep
radials-bonded-to-tower.
And any time you think about it, make sure your mental image of the setup
is a big ad-hoc transformer.
Matching a cranky L is treated in depth at "Taming the Exasperating
Inverted L" on k2av.com
<https://k2av.com/K2AV_Taming_the_Exasperating_Inverted_L.html> . See
"Method B", which uses Inv L horizontal length to adjust X to zero at
chosen center frequency. Then you vary or purchase the needed turns ratio
on a balun or isolation transformer to bring 20-35 ohms up to 50. Not using
a lumped series resonating element produces a broader SWR bandwidth.
Balun Designs' 16132 model series or equivalent, 32 to 50 ohms transform,
has been "good enough" for quite some number of these optimized L's. A
good, completely loss-remediated L at resonance ought to be somewhere 20-35
ohms. That might be higher due to loss issues that simply can't be dealt
with, for whatever reason. Robust DIY field-adjustable isolation
transformers <https://k2av.com/K2AV_AdvancedIsolationTransformers.html>
covering
input R from 23 through 46 ohms, producing output 48.5 to 52 ohms, are
described in the k2av.com articles. These transformers do not have to be
configured as isolation transformers. The k2av.com windings can be
connected in circuit as ununs.
There is good reason for including "taming" and "exasperating" in the
tuning article's title. BTDT, with many likewise sufferers among
correspondents.
The tuning article on k2av.com was written to help Inv L over FCP
installers who don't have the property for a proper radial system.
*However,* the impedance-matching detail applies to any counterpole under a
quarter-wave-ish inverted L.
Anyway, Happy Holidays and especially Good Luck getting all solved.
73, Guy K2AV
> On Wednesday, December 20, 2023 at 07:52:00 PM CST, Steve Muenich <
> srmuenich@gmail.com> wrote:
>
> I have an Inverted-L question that hopefully someone can answer for me.
>
> I installed the 160m wire to the 80 ft level on a 100 ft Rohn 45 tower
> with top mounted yagis.
> The horizontal (sort of) portion is approx 45ft long.
> The wire starts at 80 ft down approx 5 ft from tower and when it gets to
> the match box the bottom of the wire is about 10ft away from tower.
>
> I have about 50 or so 120 ft long radials.
>
> I am able to get a X=0, R=52. SWR 1.19:1 tune according to my RE Zoom using
> load shunt match at base of tower.
>
> My question is why do I need so much series capacitance (5300pf) with
> parallel inductance approx 8uh?
>
> Everything is working fine, but I am wanting to understand why I am needing
> so much series capacitance? Does this indicate an issue I need to resolve?
>
> TIA,
>
> Steve, NA5C
>
>
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