....Anyone ever done a helical wound "Slim Jim?"
yes.
Since this is a 1/2 wave
antenna with a 1/4 matching section (3/4 overall),

Why the 1/4 matching section? Seems that takes it
nearly to the full half wave length...or close
enough not to matter much...
Myself, I would probably prefer a "gamma loop"
type matching scheme, which adds no height.
To me, 16 ft tall is no more trouble than 12 ft tall
in the real world.

SNIP

The SlimJim resembles the J-Pole. The 1/4 wave section is a matching
network and it is probably not critical that it be vertical so the
height may be reduced by about 1/3. I am not sure what the folded
section does for it. but without the 1/2 wave section coming back
down from the top, the antenna is just a J-Pole.




helical winding of this form into a 3/8 overall height
(approx. 12 ft. @ 28Mhz) might be possible.
The extremely low angle of radiation (approx. 8 degrees)
would make this vertical antenna desirable... ......................

I like my antennas full size if at all possible.
I'm a radio bully. :/ MK

I am with you here, when it is possible and practical.


--
73 for now
Buck
N4PGW

In article ,
Buck wrote:

The SlimJim resembles the J-Pole. The 1/4 wave section is a matching
network and it is probably not critical that it be vertical so the
height may be reduced by about 1/3. I am not sure what the folded
section does for it. but without the 1/2 wave section coming back
down from the top, the antenna is just a J-Pole.

The Slim Jim appears to be identical to the "top wire double radiator"
J-pole which is described in Cebik's multi-part discussion of J-pole
designs: see http://www.cebik.com/vhf/jp2.html

His analysis suggests that the performance and behavior of this type
of J-pole are essentially identical to those of a single-radiator
J-pole. There are some slight changes needed in the antenna
dimensions to get it resonated properly (the coax attachment point is
a fraction of an inch higher up and the total length is a bit
different) but once those are taken into account there doesn't seem to
be any real difference in performance between this variant, a
version with two wires which are left unconnected at the top, and a
version with one wire.

At their best, they appear to be simple 1/2-wave radiators. The hype
on the HamUniverse page about this antenna outperforming "1/2wave over
1/2wave over 1/2wave colinear!" seems a bit overdone - if it's ever
true, it would be in comparison to a rather badly-done colinear!

My expectation is that one could create a helically-wound version of
any of these J-pole versions. The matching arrangement would no doubt
need some adjustment. I'd expect the gain to drop off, with more
high-angle radiation appearing, in the usual fashion as the total
length of the radiating section is shortened.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Buck wrote:
The SlimJim resembles the J-Pole.

And the Zepp.

I am not sure what the folded section does for it.

Folding a 1/4WL vertical raises the feedpoint impedance.
Folding a 1/2WL vertical lowers the feedpoint impedance.

A folded 1/2WL monopole has about half the end-fed feed-
point impedance of an end-fed single-wire 1/2WL monopole.
About 1250 ohms Vs 2500 ohms for 20m with mininec ground.

EZNEC sez the folded 1/2WL monopole has a slightly lower
resonant frequency and slightly higher gain. (+0.07 dB)
--
73, Cecil http://www.qsl.net/w5dxp

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Interesting... so electrical length is affecting gain and EZNEC supports
it--that will probably silence those who claim the physical length is all
important... ya suppose?

Regards

--
Hay, if'n ya'll cun't konstructivly partecipete in this disscusion, haw
aboot speel-checkin it fer me?


"Cecil Moore" wrote in message
...
Buck wrote:
The SlimJim resembles the J-Pole.

And the Zepp.

I am not sure what the folded section does for it.

Folding a 1/4WL vertical raises the feedpoint impedance.
Folding a 1/2WL vertical lowers the feedpoint impedance.

A folded 1/2WL monopole has about half the end-fed feed-
point impedance of an end-fed single-wire 1/2WL monopole.
About 1250 ohms Vs 2500 ohms for 20m with mininec ground.

EZNEC sez the folded 1/2WL monopole has a slightly lower
resonant frequency and slightly higher gain. (+0.07 dB)
--
73, Cecil http://www.qsl.net/w5dxp

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On Mon, 28 Mar 2005 18:21:52 -0800, "John Smith"
wrote:

Interesting... so electrical length is affecting gain and EZNEC supports
it--that will probably silence those who claim the physical length is all
important... ya suppose?

Hi "Fred,"

It will take more than supposition and superstition.

So back to rote:
The physical size in relation to wavelength dominates launch
characteristics, NOT electrical length.

73's
Richard Clark, KB7QHC

"Richard Clark"
So back to rote:
The physical size in relation to wavelength dominates launch
characteristics, NOT electrical length.
__________________

I don't know who wrote your rote, but here is part of what Terman says on
this topic in his Radio Engineer's Handbook, p 795, referring to vertical
radiators driven against ground:

"Top loading has the same effect on the field distribution in a vertical
plane as a greater height. Thus an antenna for which H = 0.45 lambda can by
suitable top loading be made to have a field distribution in the vertical
plane that is substantially the same as for a vertical wire of H = 0.6
lambda."

Or is that what you meant?

RF

"Richard Fry" wrote So back to rote:
The physical size in relation to wavelength dominates launch
characteristics, NOT electrical length.
__________________

I don't know who wrote your rote, but here is part of what Terman
says on
this topic in his Radio Engineer's Handbook, p 795, referring to
vertical
radiators driven against ground:

"Top loading has the same effect on the field distribution in a
vertical
plane as a greater height. Thus an antenna for which H = 0.45
lambda can by
suitable top loading be made to have a field distribution in the
vertical
plane that is substantially the same as for a vertical wire of H =
0.6
lambda."

=============================

No, it doesn't !

But you could stretch "substantial" (a non-engineering term) a bit
more.

You have been warned once before about quoting Terman as the Bible.
(smiley)
----
Reg.

"Reg Edwards" wrote about the elevation pattern of a loaded vertical against
ground as being ~ the same as that of a longer, unloaded vertical, per
Terman:

No, it doesn't ! You have been warned once before
about quoting Terman as the Bible.
______________

I wonder, then, what your basis is for saying so.
At least I give a source.

Terman also publishes a formula to calculate the elevation pattern of a
shortened vertical with a top-mounted capacity ring, driven against
ground -- but it's too much to post here without mathematical notation. The
formula was credited by Terman to George H. Brown from his "A Critical Study
of Broadcast Antennas as Affected by Antenna Current Distribution" published
in the Proceedings of the I.R.E.

Terman also says that inserting a coil a bit down from the top of a
shortened vertical gives results equivalent to using a top mounted capacity
ring.

RF

Reg, G4FGQ wrote:
"No it doesn`t! (Thus an antenna for which H=0.45 lambda can by suitable
top loading be made to have a field distribution in the vertical plane
of H=0.6 lambda.)"

Reg is right. Between two antennas there will always be differences.
But, as Richard Clark might say, "Does it make a Db of difference?" One
dB can easily be lost in measurement error.

Top loading has been around since at least 1909 when it was patented by
Simon Eisenstein of Kiev. Russia. See Fig 9-24 on page 9-17 of ON4UN`s
"Loe-Band DXing". Eisenstein shows current distribution on his patent
application. He gets the base current up as it might be in a full height
antenna.

I would believe what Terman wrote because I`ve never been able to
disprove anything he wrote. Now I look for my error in logic when
something of Terman`s seems wrong.

ON4UN says on page 9-29 of his 1994 edition of the Low-Band DXing book:
"Over sea-water the 5/8 wave has lost 0.8 dB of its gain already, the
1/4-wave only 0.4 dB." (It`s less than one dB).

Even a disappearingly small radiator produces radiation less than 1/2 dB
weaker than a 1/2-wave dipole, or a 1/4-wave vertical. In lossless
antennas, the only difference in radiated signal between the full length
antenna and a too-short antenna comes from the slight difference in
their patterns.

Short antennas have efficiency problems because they have low radiation
resistances. This low radiation reaistance goes not compare as well with
a given loss resistance as does the higher radiation resistance of the
full size antenna. However, great care can be taken with the too-short
antenna to minimize its loss resistance and get good efficiency.

You have only to consult the "ARRL Antenna Book" and compare a short
continusously loaded vertical`s performance with that of a full-size
1/4-wave vertical. In my 19th edition it`s on page 5-25:
"Fig 46-Helically wound ground-plane vertical. Performance from this
type of antenna is comparable to that of many full-size 1/4 vertical
antennas."

In 1949, I worked in a transmitting plant where two stations, KPRC, 950
KHz, and KXYZ, 1320 KHz, shared the same transmittinng tower. Both
stations had identical RCA 5-C, 5 KW transmitters. Regional coverage was
almost identical despite many more degrees in the tower at 1320 KHz than
at 950 KHz.

One of the operators at the stations was a ham. He was J.L. Davis,
W5LIT. J.L. had a new 1949 Ford with a cane pole bolted to the rear
bumper. The pole was wound nearly end to end with enameled wire to serve
as antenna for his mobile ham rig. He had no top hat at the tip of his
antenna, so sometimes when he was talking a high voltsage corona
discharge would plume from the top of his antenna. Very impressive
though no help to his QSO..

Bill Orr writes on page 78 of "Vertical Antennas":
"A helix length of about .05 wavelength or more provides good results as
a substitute for a full size quarter wavelength vertical antenna."

It worked for W5LIT.

Best regards, Richard Harrison, KB5WZI

John Smith wrote:
Interesting... so electrical length is affecting gain and EZNEC supports
it--that will probably silence those who claim the physical length is all
important... ya suppose?

The important point is that the web page claim of bettering
three end-to-end 1/2WL dipoles misses by about 3 dB and a few
degrees on TOA.
--
73, Cecil http://www.qsl.net/w5dxp


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