Re: Topband: Inverted-L question

[Guy Olinger K2AV]

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"
 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 

Topband: Merry Christmas...

[Gene Smar via Topband]

... to all who celebrate. 
73 de Gene Smar AD3F 

Sent from my Radio Shack TRS-80 model 100 laptop
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Topband: Merry Christmas

[Michael Rutkaus]

To all, nice to know you,

Mike

K4QET

On Sun, Dec 24, 2023, 12:00 PM  wrote:

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> Date: Sat, 23 Dec 2023 18:34:17 -0500
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> Merry Christmas and Happy Holidays to all of the Topband posters.
> Since installing an Inv-L about a year it's been fun to discover
> you can operate on 160 meters from a small lot.
>
> I hope everyone has a safe and prosperous Holiday Season.
>
> Karin Anne Johnson  K3UU
> Palm Harbor, Florida
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