Why would anyone use just 43ft of vertical without at least top
loading the vertical section.?
Without considering maximizing the antenna efficiency, I don't think
considering losses in matching coils is valid.
Not many people know this but my 160m vertical is but 51ft tall, top
loaded but has sufficient ground radials to bring the overall losses down to
a low figure. My overall point is the equation for efficiency relies on
minimising ground losses irrespective of antenna height as per Brown Lewis
and Epstein. This should be the object of anyone seeking to improve their
160m antenna.
73 Clive GM3POI
-----Original Message-----
From: topband-bounces@contesting.com [mailto:topband-bounces@contesting.com]
On Behalf Of Guy Olinger K2AV
Sent: 31 July 2012 19:44
To: Charles Damico
Cc: TopBand List
Subject: Re: Topband: K2AV FCP with 43' vertical
Hi, Charles,
The short, short twitter answer: 43 feet is too d*mn short for 160. Do
something else.
Merely short answer: Yes, you can use an FCP. But...
Hate to say it's the wrong question, sounds too much like a put down which
I don't intend. The real question is why use a 43 foot vertical with that
antenna's hypersensitivity to loss issues on 160. 43' on 160 is a very
short antenna, electrically. A model of it with four full size radials and
all sources of loss removed shows a radiation resistance of 2.6 ohms (two
point six), and a feed current in the neighborhood of 25 amps. Over dirt
it's possible you are down TWO S units, depending on exactly what you meant
by four radials, plus other loss factors. Do you have to use the 43' on
160? If you really do, that's one direction. If you can substitute an L,
that's another direction.
Long answer:
To answer your first question, you would be the first that I know of
considering this combination. Rare I think, because there are such better
alternatives for a small lot without all the downsides. The downsides are
fairly extreme, and why so is worth a few paragraphs.
"Very short" solutions have certain characteristics, a natural very low
radiation resistance, very high current levels, and a very narrow SWR
bandwidth. The way to assess these is to model an lossless environment.
We'll
use 43' over four 125' raised radials at 8 feet, made of zero resistance
conductors and over salt water to eliminate ground losses. Magically in
the model we have a transmitter right at the feed point, which has a very
large matching range to deal with complex impedances with low single digit
resistance and up to 1000 ohms capacitive reactance. Also have other magic
in models, wires that support themselves above ground, etc, but I
digress... We'll be using NEC4 engine in W7EL's EZNEC Pro, and the
Sommerfeld ground approximation method aka "high accuracy". There is
controversy about accuracy of currently available modeling programs for
sparse radials like yours, a really good case that ground loss is
UNDER-estimated, but again I digress... Going to try and do this in a
manner that skirts those issues.
At 1500 watts on the lossless antenna, the peak current on the vertical is
24 amperes, and the feed impedance is 2.6 ohms -j986 reactance. Not a
typo, that's two point six ohms. The peak gain is 4.68 dBi. Eliminating
the capacitive reactance of the short radiator takes 86 uH to get 2.6 +j0.
86 uH is quite the large coil, but in this example it's made of
super-conducting wire. The 2:1 SWR bandwidth is 3 kHz (yes, that's THREE
kHz). Now let's start to inject reality, dirt, etc. Any increase in
resistance above 2.6 ohms is ALL due to loss. Any broadening of the 2:1
SWR bandwidth of 3 kHz is due to loss.
Let's change the ground medium in the model from "over salt water" to
EZNEC's "average" dirt, and change the conductors to copper. The feed R
goes to 4 ohms. The max gain drops to -0.72 dBi. The 2: 1 SWR bandwidth
goes to 5 kHz. The peak current drops to 19.2 amps.
If we add a conservative 5 ohms for the effective series resistance (ESR)
of the huge loading coil necessary to provide 86 uH inductance to tune the
antenna, the current drops to 12.8 amps, the gain to -4.2 dBi (!!!!), the
feed R is up to 9 ohms, and the 2:1 SWR bandwidth is a semi-usable 10 kHz.
In the coil, the current squared times 5 ohms is around 800 watts
dissipated in the coil. The heat loss in the coil at QRO, particularly
inside a protective enclosure that can trap heat, will be hot enough to
soften the plastic bars used to support coil stock and cause them to get
"gravity droops", or be destroyed, perhaps set the whole thing on fire. I
have destroyed similar components myself in years past, unaware of the
level of current. We are the kings of burning things up, Jack and I, but I
digress... At 100 watts, the dB loss is the same but the power dissipation
is not large enough to signal the degree of loss by deforming or destroying
things.
If we add in Guy's personal pessimistic adjustment for NEC's
underestimation of ground losses at MF in these kinds of sparse radial
situations (only 4 elevated radials) , the gain drops toward -6 dbi. Ten
dB below perfect, and more than an S-unit less than easily obtainable with
an L over an FCP.
The killer is the coil. You could do something with silver plated 1/4 inch
copper tubing to get the ESR of the coil down, to where the losses in the
dirt were controlling. Yes, the FCP will improve things, and raise the
feed Z some, but you are now running the isolation transformer at current
levels we haven't had running for two years watching results. 43 feet and
160m takes you into the land of compromises. You expect these kinds of
losses in low band mobile installations, you don't expect fixed stations to
be constrained in this manner.
73, Guy
On Sat, Jul 28, 2012 at 11:22 AM, Charles Damico
<frontsight41@yahoo.com>wrote:
> I would like to hear from anyone that has used or considered using the
> K2AV FCP with a 43' vertical radiator. I have a DXE 43' vertical with 4
> elevated radials and have fair results on 80 and 160 meters. Any
> info/thoughts on the subject are welcome.
>
> Thanks,
>
> Chuck AJ1E
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
>
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