RadioBanter - j-pole 5/8 wave

wrote:
On Oct 6, 3:19 pm, "Jimmie D" wrote:

If I remember correctly the oft quoted theorotical gain of a 5/8ths is with
a groundplane that extents to infinity.
5/8ths with sloping 5/8th radials are begining to perform more like a
ceterfed collinear.

Jimmie

Exactly. That is what you want. A 5/8 element is only half an
antenna,
and the other half wants to be a 5/8 too to work properly. All a 5/8
GP
is , is half of an EDZ. You would never want to run only half of an
EDZ,
or use a 1/4 wave leg on one side, etc.. Like I say, it's a
perversion.
The 1/2 wave is the only single element that can be a proper complete
antenna on it's own. Or according to the laws of NM5K anyway... :/
And of course, even that "complete" antenna needs decoupling from
the feedline to live up to it's full potential.
The success of a 5/8 antenna varies a lot across the spectrum.
On 2m, I hate em... Not a good pattern for VHF, unless you use the
collinears, etc..
But... On 10m, it's my favorite vertical.. I've done careful tests
over
the years, and I've never had any other type single element beat it.
But... I was using 3/4 wave radials the last time I did do testing
on all those.
In one test, I started with a 1/4 wave GP with sloping radials.
Worked ok..
Then I built a 1/2 wave whip, and fed as a ringo.
I saw an improvement over the 1/4 GP, even with no decoupling.
So, I then decided to add a decoupling section, using a 1/4 wave
section of the feedline below the feed, and attached to a set of
1/4 radials.
This improved the antenna a good bit. It was really humming
along at that point. I used mainly stable local signals to test
any improvements. When I added the decoupling to the halfwave,
I noticed a bit less bandwidth as far as the antenna, but the
performance was more stable, and I assume would be about the
same no matter what line length I used, etc..
Anyway, I had the half wave working about as well as possible I
think.
Then I built a 5/8 radiator, and used 4 sloping 3/4 wave radials.
Guess what? It beat the decoupled half wave by an easily noticed
margin on the low angle space/ground wave paths I was using to
test. The other stations were spread across town in various
locations, some 30-40 miles away.
If the low angle performance of the 5/8 was crippled, I sure
didn't see it here on 10m. It still had enough gain at low angles
to beat anything else I could try.
BTW, all those antennas were mounted at the same 36 ft height.
I also ran a 5/8 GP on 17m for a while. That antenna beat every
other antenna I had that could be tuned for 17m. IE: wire
dipoles, etc..
I've seen a load of people use the usual "perverted" 5/8 GP
on the CB band. Even with the short 1/4 radials, I never saw
one of those lose to a 1/4 wave GP. On the average CB meter, '
the change from a 1/4 GP to the 5/8 GP was normally good
for about 2 S units on the average CB , if you were talking
across town a ways. I've seen this too many times for it to
be a fluke of nature. This goes back to the early 70's..
Anyway, I like 5/8 verticals on the HF bands. But myself,
I don't use the short 1/4 wave radials.
As far as 5/8 whips on cars, just depends on the path.
Here in Houston, with the flat terrain, a 5/8 will generally
beat a 1/4 wave by a noticable amount, and will usually
have less picket fencing.
But in those cases, the metal under the antenna is fairly
large vs wavelength. IE: a 5/8 at 146 mhz is about 48 inches.
So it doesn't take a huge vehicle to give a decent ground
plane if the antenna is on the roof or trunk.
Myself, I think most mobile 5/8 whips work better than
elevated 5/8 GP's used on the same band.
In some areas, the terrain will favor using the 1/4 wave
though.
Like I say, the usual 5/8 with 1/4 wave straight radials
is generally the pits on 2m, unless some strange quiver
in the force kicks in, like say in phase feedline currents,
etc. And thats fairly rare in the real world. It can happen
though..
A properly decoupled dual 5/8 collinear on 2m is a stout
antenna and will be hard to beat unless you take drastic
measures. Once I get to that point, I switch to a yagi if
I want to do better. A 3 el yagi will pretty much smoke
any practical vertical.
Anyway, the performance of 5/8 antennas varies quite
a bit depending on where you are using them at.
The 5/8 with 3/4 radials I used on 10m was pretty
stout overall. It was the best out of all the usual
lengths.
The real world and the modeling programs don't always
exactly jive..
IE: On Cevics page, he mentions that in his modeling
tests, he seems to think the 5/8 GP was generally
not worth the trouble on the upper HF bands.. IE: 10m
I know from real world use, it is worth it. No doubt
in my mind. But I also know there are much better
radial lengths than the straight 1/4 wave.
BTW, I agree with one thing he mentioned..
Sloping 1/4 radials with the 5/8 is the pits... You want
1/4 wave radials straight out if they are under a 5/8
whip. Sloping them will really whack out the pattern.
But sloping radials are no problem under a 1/4 wave
whip. Now, on the other hand, sloping 3/4 or 5/8
radials under a 5/8 whip are ok.
MK

This gives me a new way of looking at the Isopole antennas. I am
begining to think the upper sleeve is not there for isolation but
rather it is a radiator and the bottom sleeve is there to decouple it
from the mast and feedline.

Jimmie