wrote in message
...
On Friday, February 14, 2014 11:21:01 PM UTC-6, Brian Reay wrote:

BIG SNIP



What in particular do you have an issue with, please?

little snip

Brian

G8OSN/W8OSN

Well, I suppose most is OK, but there are a few problems.

BIG SNIP

I had a question about his j-pole analysis. He says "A J-pole, like ANY end
fed antenna, needs radials, a counterpoise or ground plane to work
properly."

I've never seen this anywhere else, even as a suggestion or hint. I can
envision places to fit one but what would it do? As built, both the pattern
and the impedance already seem to be what I want.

Thoughts?

"Sal"

On Saturday, February 15, 2014 3:27:15 PM UTC-6, Sal wrote:
As built, both the pattern
and the impedance already seem to be what I want.

And it already has a counterpoise. The counterpoise is where the current flowing on the inside of the coax shield goes when it leaves the coax. The J-pole has a radiation pattern of a 1/2WL vertical, i.e. the lower 1/4WL doesn't radiate (much) and is part of the matching section and counterpoise. Quite often, the outside of the coax braid is also part of the counterpoise.
--
73, Cecil, w5dxp.com

"W5DXP" wrote in message
...
On Saturday, February 15, 2014 3:27:15 PM UTC-6, Sal wrote:
As built, both the pattern
and the impedance already seem to be what I want.

And it already has a counterpoise. The counterpoise is where the current
flowing on the inside of the coax shield goes when it leaves the coax. The
J-pole has a radiation pattern of a 1/2WL vertical, i.e. the lower 1/4WL
doesn't radiate (much) and is part of the matching section and counterpoise.
Quite often, the outside of the coax braid is also part of the counterpoise.

-----ooooo-----
Cecil,

I read with interest your critiques of the article. On the basis that
you mentioned eznec, I wonder of perchance you are a professional?

Gareth

On Saturday, February 15, 2014 5:46:21 PM UTC-6, gareth wrote:
I wonder of perchance you are a professional?

I've been a ham for 60 years and worked as a EE for 38 years so you might say I am 2/3 professional.
--
73, Cecil, w5dxp.com

"W5DXP" wrote in message
...
On Saturday, February 15, 2014 5:46:21 PM UTC-6, gareth wrote:
I wonder of perchance you are a professional?

I've been a ham for 60 years and worked as a EE for 38 years so you might
say I am 2/3 professional.

44 and 42 for me, but mainly diverted into software engineering for 96% of
that time.

"W5DXP" wrote in message
...
On Saturday, February 15, 2014 3:27:15 PM UTC-6, Sal wrote:
As built, both the pattern
and the impedance already seem to be what I want.

And it already has a counterpoise. The counterpoise is where the current
flowing on the inside of the coax shield goes when it leaves the coax. The
J-pole has a radiation pattern of a 1/2WL vertical, i.e. the lower 1/4WL
doesn't radiate (much) and is part of the matching section and counterpoise.
Quite often, the outside of the coax braid is also part of the counterpoise.
--
73, Cecil, w5dxp.com

Thanks, that's what I thought, too. The nature of current flow on the
outside of the coax is the reason so many construction articles mention
winding a few turns of the coax into a choke, not far* from the antenna. I
don't always do it but I probably should.

* But what distance is "not far"? A quarter-wave comes to mind, so a
measurement is called for. Hm-m-m ... is the propagation velocity on the
outside of the coax the same as the inside? My head spins just a little.

"Sal"

On Saturday, February 15, 2014 6:59:25 PM UTC-6, Sal wrote:
* But what distance is "not far"? A quarter-wave comes to mind, so a
measurement is called for. Hm-m-m ... is the propagation velocity on the
outside of the coax the same as the inside?

Common-mode chokes work best at the high current points so "not far from the feedpoint" of a 50 ohm antenna. Two chokes, one at the antenna and one 1/4WL down the feedline, work well. The velocity factor of a common-mode signal on the outside braid of the coax is fairly close to 1.0 only slowed down by the outside insulation layer.
--
73, Cecil, w5dxp.com

"W5DXP" wrote in message
...
On Saturday, February 15, 2014 6:59:25 PM UTC-6, Sal wrote:
* But what distance is "not far"? A quarter-wave comes to mind, so a
measurement is called for. Hm-m-m ... is the propagation velocity on the
outside of the coax the same as the inside?

Common-mode chokes work best at the high current points so "not far from the
feedpoint" of a 50 ohm antenna. Two chokes, one at the antenna and one 1/4WL
down the feedline, work well. The velocity factor of a common-mode signal on
the outside braid of the coax is fairly close to 1.0 only slowed down by the
outside insulation layer.
--
73, Cecil, w5dxp.com

Thankew!

"Sal"

On Sat, 15 Feb 2014 13:27:15 -0800, "Sal" salmonella@food
poisoning.org wrote:

I had a question about his j-pole analysis. He says "A J-pole, like ANY end
fed antenna, needs radials, a counterpoise or ground plane to work
properly."

I've never seen this anywhere else, even as a suggestion or hint. I can
envision places to fit one but what would it do? As built, both the pattern
and the impedance already seem to be what I want.

Thoughts?

I agree with the author (Terry Graves K7FE).

The rule-of-thumb is that an end fed antenna requires a shield ground
to act as a counterpoise and to keep the coax from radiating. The
J-pole (and Zepp) antennas are not exactly end fed, but are close
enough. A possible exception to the rule is a 1/2 wave end fed
antenna, which allegedly does not require a ground plane. I'm
undecided on that point.

However, a J-pole (or Zepp) is not a 1/2 wave antenna. The driven
element is a 1/4 wavelength long, and therefore DOES require a ground
plane. This article covers the point (and more):
http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm
Quoting:
Summary End-feds Without Grounds
ANY END-FED ANTENNA REQUIRES A LARGE GROUNDPLANE OR OTHER
EXTRAORDINARY ISOLATION METHOD OR METHODS TO PREVENT FEEDLINE
OR MAST COMMON MODE CURRENTS!
This is true for 5/8th waves, Zepp antennas, R7's, R5's, or
even common J-poles. End-feeding antennas is bad news unless
you have a large well-established ground at the feedpoint.
Even 1/4wl groundplanes have common mode problems. When I
designed a commercial 1/4 wave groundplane with four 1/4 wave
long radials, I had to insulate the radials from the mast and
isolate the coax shield from the mast and radials with a 1/4
wave stub that formed a choke balun. Without the decoupling,
I could change SWR simply by changing mast or feedline grounding.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Sat, 15 Feb 2014 19:58:10 -0800, Jeff Liebermann
wrote:

http://www.cvarc.org/tech/antenna_myths/antenna_myths.pdf
http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm

Not really a criticism, but more of an oddity. Starting on Pg 10, the
author shows that a J-Pole driving the LONG element has 2.37dBi gain,
while the same antenna driving the SHORT element has -3.17dBi gain.

I've never really looked as which way is the correct way to connect
the coax cable. I also don't have any J-Poles around the house.
Skimming the available photos:
https://www.google.com/search?tbm=isch&q=j-pole
I find a mix of methods. Most seem to do it the right way, but there
are plenty doing it wrong.
http://forums.radioreference.com/scanner-receiver-antennas/208290-j-pole-discrepancy.html
http://wmarc.wildmidwest.org/slide_shows/J-Pole_Antenna_Build/images/GEN_3764.jpg

This one is interesting because the two drawings show the correct
wiring, but the photograph shows it built backwards.
http://www.iw5edi.com/technical-articles/144-430-dual-band-jpole-antenna
Sigh.

I hate to admit it, but I think I've built them backwards over the
years. Maybe that's why J-poles have such a lousy reputation and why
I think they suck?

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558