Well, If I'm getting it right, the 43' feet is being used on all
bands, and there are "approved by the garden committee" kinds of
issues. I don't argue with your general take in the least, but we are
working in severely restrained circumstances in this thread, not
laying out universal rules for the masses. For sure, some of the
proper issues you list will not be within reach in this case, at least
not without family or HOA repercussions.
It IS possible to put up black wire in trees and construct a system
that can't be seen from the street. One fellow (call him Danny Boy to
protect anonymity) put up poles supporting an FCP, and tortuous
threading of black insulated #12, and a matching box, all with
camoflauge painting. Invisible from the street even if you know where
it is. Wifey came home expecting to be upset, Danny Boy had been
pushy about it all, and she could not see it from the street even when
told where to look. Neighborhood folks have been over since, Dinner
out on the patio, barely 50 feet from the thing in full view, and
noone noticed. (Ah, there is a REASON why camouflage works.)
I don't think everybody has Danny Boy's chutzpaz. So I take these 43
foot inquiries seriously, rather than suggesting a divorce and moving
away. Have to raise all the issues and possibilities to enable the
fellow that's living on the scene. In the end, only he knows what can
be gotten away with. Scaring about high currents and losses gives him
reasons to involve in his choice.
73, Guy.
On Tue, Jul 31, 2012 at 5:02 PM, Clive GM3POI <gm3poi2@btinternet.com> wrote:
> 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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