Voltage feeding a VHF yagi
 

wrote:
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.

Look at what the J-pole really is.

1.) You have a half-wave end-fed antenna. There has to be as much
common mode current leaving the end of that point and flowing down the
feedline as there is flowing out onto the antenna at that point. There
isn't any exception to this rule.

2.) While that current may be small with a perfect half wave, it is
never zero. It gets worse fast of the antenna is not 1/2 wl long
electrically, or if it is thick.

3.) Now you have a 1/4 wl stub feeding that half-wave on the end. If
you perfectly floated that 1/4 wl stub, common mode current in the stub
would DECREASE as you move away from the feedpoint. But if you ground
the 1/4 wl stub, current common mode INCREASES as you move away from
the stub. This is why end-fed Zepps and J-poles model very good when a
perfect ground independent current source is used to feed them.
Unfortunately we can't do a perfect ground independent feedpoint in the
real world, so depending on the CM impedance the amount of pattern
distortion will be all over the place.

This is why Zepps, antennas that are really just the same as a J-pole,
are notorious for RF in the shack. The lack of feedlines in models are
why people who do not include the feeder or feedline matching device to
the 1/4 wl closed stub conclude they aren't so bad.

Why would anyone go through all that bother to complicate the feed
system in a Yagi is beyond me, when there are a half dozen easy
solutions that were mentioned here.

73 Tom

Voltage feeding a VHF yagi
 

wrote:
1.) You have a half-wave end-fed antenna. There has to be as much
common mode current leaving the end of that point and flowing down the
feedline as there is flowing out onto the antenna at that point. There
isn't any exception to this rule.

It certainly isn't that simple in a distributed network.
The currents into the end of a half-wave section are certainly
unbalanced at X and Y but the current amplitudes are pretty low.
5 watts into 5000 ohms is only about 30 mA. 1/4WL back at the
shorted matching section at ++, the currents have to be close to
balanced because of the short. The coax connection at A and B is
closer to the short than it is to to X and Y. The current down
the coax braid at B does NOT have to be equal to the current at
the feedpoint at X since they are a good percentage of 1/4WL
different in location. And common-mode current can form standing
waves so common-mode current maximums and minimums exist depending
upon location on the feedline. The lumped circuit model strikes
(out) again.

X
+--A----------------------------
|
+--B--------Y
--
73, Cecil http://www.qsl.net/w5dxp

Voltage feeding a VHF yagi
 

Jeff wrote:

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

I think normally the effect is fairly small. But I suppose it can vary
per the position, and mast length, etc.. I guess it's all how picky
you are about the gain/pattern. For moonbounce, etc, it might be worth
worrying about. But for just a general purpose FM beam, I probably
wouldn't worry too much. Unless the mast was a certain length
to become resonant, it should pretty much be ignored by the antenna
for the most part. Seems to me, right in the middle of the two
midpoint elements would be about best, but I've never modeled
other positions. I have seen quite a few people run vertical yagis
with the midpoint mast mount with no serious problems.
I think any small loss of performance would be less than trying to
voltage
feed thats for sure. You'll see lots of antenna ad's with pictures of
vertical beams mounted on masts in that manner, so it shouldn't be
too bad. I agree. There should be little difference in using a metal
mast, or plastic, if the feedline shield is in place on both. I'd just
as
soon use the metal mast I think. It would look fatter with the extra
metal mast, but I don't know if that would be a plus or a minus...
MK

Voltage feeding a VHF yagi
 

In message .com,
" writes
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?

There's an example of a 4ele 2m J-fed yagi in the MMANA files. It has a
gain of 7dBd. Free space pattern is skewed up by 1 degree, but a real
ground will do that anyway.

I don't know of any commercial versions.

Brian GM4DIJ
--
Brian Howie

Voltage feeding a VHF yagi
 

Cecil Moore wrote:
It certainly isn't that simple in a distributed network.
The currents into the end of a half-wave section are certainly
unbalanced at X and Y but the current amplitudes are pretty low.
5 watts into 5000 ohms is only about 30 mA. 1/4WL back at the
shorted matching section at ++,

Not that it ever does any good to try to get you to think about what
you are saying, but that is nonsense Cecil.

The short doesn't affect CM currents. The 1/4 wl line can act as
current step up if the far end is grounded.

Also, reach back in your rear and pull out another impedance number.
The impedance value you grabbed from there is for a very thin wire
compared to length, like an end-fed HF antenna. Tubing is alot lower on
the end Cecil OM.

Before going off on another Cecil-knows-best event and destroying a
thread to make it all about you, run the model. Six meters, yagi,
tubing. Not 40 meters and #16 wire.

Voltage feeding a VHF yagi
 

wrote:
The short doesn't affect CM currents.

I1
+-----A------------X-----------------------------
|
+-----B------------Y
I2

I1 and I2 are common-mode. What do you suppose will
happen at the short?
--
73, Cecil http://www.qsl.net/w5dxp

Voltage feeding a VHF yagi
 

Cecil Moore wrote:
wrote:
The short doesn't affect CM currents.

I1
+-----A------------X-----------------------------
|
+-----B------------Y
I2

I1 and I2 are common-mode. What do you suppose will
happen at the short?
--
73, Cecil http://www.qsl.net/w5dxp

I know what happens.

The problem is what YOU are supposing happens.

Voltage feeding a VHF yagi
 

wrote:
Cecil Moore wrote:
wrote:
The short doesn't affect CM currents.
I1
+-----A------------X-----------------------------
|
+-----B------------Y
I2

I1 and I2 are common-mode. What do you suppose will
happen at the short?

I know what happens.
The problem is what YOU are supposing happens.

I'm supposing Kirchhoff's laws apply. What are you
supposing? How is the above different from link
coupling?
--
73, Cecil http://www.qsl.net/w5dxp

Voltage feeding a VHF yagi
 

wrote:
Cecil Moore wrote:
coax center conductor
|
I1 |
+-----A------------X-----------------------------
|
+-----B------------Y
I2 |
|
coax braid

Kirchoff's laws apply, but you're forgetting one conductor.

------A+B----coax braid-------------|

We are talking about what happens at the short on the left,
not what happens at points A or B where the coax is connected.
W8JI said the short doesn't affect the common mode current.
Seems to me, the short has approximately the same effect on
common mode currents as does a link coupling.

The coax center conductor is connected at 'A' and the braid
is connected at 'B'. Do you agree with W8JI that the common
mode current at 'X' must equal the common mode current at 'B'?
--
73, Cecil http://www.qsl.net/w5dxp

Voltage feeding a VHF yagi
 

Instantaneous sum of the common mode currents at B (you can give it an
overall minus sign if you want, but it's right for common mode):

Current flowing toward B from X + Current flowing toward B from Y -
current flowing out from B via coax shield - current flowing out from B
via coax center conductor.

All the common mode current on the coax will flow on the outside. So
current flowing out from B via coax center conductor is zero.

So current flowing on the coax braid = Current flowing from X to B +
Current flowing from Y to B.

Points A and B are one point except for the reactance of the short
connecting them.

Dan