Voltage feeding a VHF yagi
 

Anyone ever get around the VHF FM vertical yagi feedline routing
problem by voltage feeding the bottom of the driven element with a stub
like a J-pole? A quick EZNEC run seems to indicate that it's not a bad
idea.

So it seems like it might work OK... but I wonder if anyone has
actually done it. Would you expect a coaxial stub to work better than
a parallel wire stub in terms of preserving the pattern of the yagi?

Any commercial products doing this?

73
Dan
N3OX
www.n3ox.net

Voltage feeding a VHF yagi
 

In article .com,
wrote:

Anyone ever get around the VHF FM vertical yagi feedline routing
problem by voltage feeding the bottom of the driven element with a stub
like a J-pole? A quick EZNEC run seems to indicate that it's not a bad
idea.

I do recall seeing one design for a "Jagi" - a three-element vertical
beam in which all three elements were of the copper-pipe J-pole style
of construction. It was reported to work pretty well. I'm afraid I
no longer have a reference to the design... sorry.

So it seems like it might work OK... but I wonder if anyone has
actually done it. Would you expect a coaxial stub to work better than
a parallel wire stub in terms of preserving the pattern of the yagi?

My guess is that either could work. For the parallel-wire/pipe stub,
it might be best to use the time-honored trick of making a half-wave
coaxial balun, attaching it a bit higher than you'd do for a direct
coax attachment, and insulate the bottom of the stub from its mast.
Adding a ferrite or coiled-coax choke to the feedline might also be a
good idea.

--
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!

Voltage feeding a VHF yagi
 

wrote:
Anyone ever get around the VHF FM vertical yagi feedline routing
problem by voltage feeding the bottom of the driven element with a stub
like a J-pole? A quick EZNEC run seems to indicate that it's not a bad
idea.

So it seems like it might work OK... but I wonder if anyone has
actually done it. Would you expect a coaxial stub to work better than
a parallel wire stub in terms of preserving the pattern of the yagi?

Any commercial products doing this?

73
Dan
N3OX
www.n3ox.net

I guess I'm missing it, but what problem is there
running the feedline to the normal center fed yagi?
No matter if against a mast, or mounted on a mast
with the center of the boom attached to the mast, I
don't see a problem. I do it all the time.
No matter what polarization, I tape the coax to
the mast, and then tape it to the boom to the feedpoint.
I'd much rather run the yagi in the normal manner.
Trying to voltage feed one sounds like a common
mode nightmare , unless drastic precautions were
taken. And I probably don't have to mention how
radiation from a feedline can skew a antenna pattern
real nicely... :(
MK

Voltage feeding a VHF yagi
 

I guess I'm missing it, but what problem is there
running the feedline to the normal center fed yagi?
No matter if against a mast, or mounted on a mast
with the center of the boom attached to the mast, I
don't see a problem. I do it all the time.
No matter what polarization, I tape the coax to
the mast, and then tape it to the boom to the feedpoint.
I'd much rather run the yagi in the normal manner.

This is a problem that is particular to vertically polarized yagis that are
mounted with a pole going between the elements. The pole has to come up
parallel to the elements to the boom. Some people overcome this by using
non-conductive pole, BUT what do you do with the feeder, if you tape it to
the pole you might just as well have used a metal pole!!!

However, as I said in my previous post, my simulations seem to show that
with careful positioning the effects can to quite small

73
Jeff

Voltage feeding a VHF yagi
 

wrote:
Anyone ever get around the VHF FM vertical yagi feedline routing
problem by voltage feeding the bottom of the driven element with a stub
like a J-pole? A quick EZNEC run seems to indicate that it's not a bad
idea.

So it seems like it might work OK... but I wonder if anyone has
actually done it. Would you expect a coaxial stub to work better than
a parallel wire stub in terms of preserving the pattern of the yagi?


Why would you want to take a directional antenna and feed it with one
of the poorest feed systems possible?

J poles and Zepps have terrible problems with common mode feeder
current because the feedline has an UNbalanced antenna element fed by a
BALanced stub that is fed by an UNbalanced coaxial line.

If you are worried about getting the feedline down an insulated mast,
offset the yagi out 5 feet or so on a cross arm or decouple the feeder
with a self- resonant choke. You could also use a sleeve or skirt
decoupled dipole. Any of these would be at least as good as an
end-feed, and more likely and easier to make much better.

73 Tom

Voltage feeding a VHF yagi
 

I forgot to say you could offset the coax with a crossarm out from an
insulated mast, not offset the Yagi. With a metal mast you would
offset the Yagi.

Voltage feeding a VHF yagi
 

Jeff wrote:

This is a problem that is particular to vertically polarized yagis that are
mounted with a pole going between the elements. The pole has to come up
parallel to the elements to the boom. Some people overcome this by using
non-conductive pole, BUT what do you do with the feeder, if you tape it to
the pole you might just as well have used a metal pole!!!

However, as I said in my previous post, my simulations seem to show that
with careful positioning the effects can to quite small

This problem has been investigated very carefully by moonbouncers who
want to use selectable horizontal/vertical polarization.

The best solution from the electrical viewpoint is to mount the yagi on
a fibreglass mast and route the feedlines out the rear, but this is not
mechanically practical for very large steerable arrays. Simulations have
found that metal masts in the plane of the elements can be used, and
feedlines can follow the same route, if - BUT ONLY IF - the yagi is
suitable and it's done at the right place along the boom. Under these
conditions, the penalty (in terms of forward gain) can be less than 1dB.

However, only certain yagi designs will be suitable, because the support
point must also be close to the mechanical centre of gravity. These
electrical and mechanical requirements may conflict, so I strongly agree
with Jeff that the effect of the mast and feedline will always need to
be simulated and computer-optimized. There is no way to guess at this
one.


Coming back to the original question about a J-pole feed for a
vertically polarized yagi, this technique was used in the 1950s-60s by
the British company J-Beam for TV antennas around 50MHz. The driven
elements that I remember were not actually J-shaped, but used a coaxial
decoupling sleeve that also served as a mast. The bottom end of the
sleeve was typically clamped to a chimney.

It worked... but in those days, nobody really understood how well or
poorly it worked.


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Voltage feeding a VHF yagi
 

I havent but Mr Cebik talks about it on his website;

http://www.cebik.com/vhf/jp4.html

Cheers Bob VK2YQA

wrote:

Anyone ever get around the VHF FM vertical yagi feedline routing
problem by voltage feeding the bottom of the driven element with a stub
like a J-pole? A quick EZNEC run seems to indicate that it's not a bad
idea.

Voltage feeding a VHF yagi
 

Bob Bob wrote:
I havent but Mr Cebik talks about it on his website;

http://www.cebik.com/vhf/jp4.html

Cheers Bob VK2YQA

wrote:

Anyone ever get around the VHF FM vertical yagi feedline routing
problem by voltage feeding the bottom of the driven element with a stub
like a J-pole? A quick EZNEC run seems to indicate that it's not a bad
idea.

The problem with many models is people just stick a source at the
feedpoint and assume it tells them something about how the antenna
works in the real world.

A model has a PERFECT source that is like using a perfect balun and a
transparent feedline. I can build a very feedline critical antenna with
horrible balance issues, one that will never work in the real world,
and make it behave quite well in the model.

I would never use a J-pole feed on a Yagi, except perhaps in a model.

73 Tom

Voltage feeding a VHF yagi
 

Thanks for the responses.

Tom, I didn't know that the feedline decoupling issues with J-poles
were so bad.

I'd be interested in more information about why it's such an issue.
Would a sleeve stub work better than an open wire stub? It's certainly
stupid to make the feedline MORE of an issue.

I like the sleeve/skirt decoupled dipole idea.

I hadn't thought about routing the feedline out on a sidearm. I was
thinking of doing that with the antenna, but with a large yagi it would
be a problem. Putting the feedline out a few feet and then dropping it
down to a lower point on the mast could certainly work.

As far as putting the feedline in the plane of the elements, understood
on the modeling and placement issues.

If one were to do this, would it help to have ferrite beads all along
the coax from the feedpoint to some distance down the mast? Of course
good feedline decoupling is important for any directional array, but I
imagine that a wire with a string of ferrite beads on it is pretty much
invisible to RF... is this right?

Maybe a long bead balun dropping away in between elements would be a
good way to go with no sidearms?

Thanks for the responses; I'd be interested in further ones.

73
Dan