Long Vee Beam question
 

I'd like to put up a 40 to 60 wavelength (per leg) vee beam for 144
Mhz. It would be 12 feet off the ground and unterminated. My hope is
that the angle between the wires can be smaller than normal becasue
the legs are relatively long.

How much gain (dbd) should I expect and about what take off angle will
I have?

It will be for transmitting only. With the transmitter located at the
feedpoint, is there any need for exotic feed methods?

What type of matching do I need to feed it with a 50 ohm output solid
state power amp?

Thanks,

Art

KY1K at pivot dot net


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On Wed, 07 Jul 2004 17:44:58 -0400, Albert wrote:

I'd like to put up a 40 to 60 wavelength (per leg) vee beam for 144
Mhz. It would be 12 feet off the ground and unterminated.
....
What type of matching do I need to feed it with a 50 ohm output solid
state power amp?

Hi Art,

Too many details lacking. 40 to 60 covers too much turf (beyond the
pun), you neglect how wide it will be at the ends too. I ran a couple
of tests to see the gain from 17 to 22dBi and the Z anywhere from 200
Ohms to 2000 Ohms. One thing to consider is the "law of diminishing
returns." The gain for a much smaller V is not much different from
the giant one (think 3dB for each doubling - as a shorthand, I am sure
this fails long before the 20, much less 50th, wavelength is reached).

73's
Richard Clark, KB7QHC

OK, thanks Richard.

How about this.

30 wavelengths per side, average conducting ground, 12 feet above
ground.

What angle do I need for best forward gain, what will the takeoff
angle be at that angle?

If I can get the entire antenna up to 20 feet above ground, how much
better is the performance?

Is there any way for me to do modelling myself without burdening you
or someone with a modeling program? I am not aware of free software
that will model rhombics or vee's.

Thanks,

Art

ky1k at pivot dot net

PS: Some additional info. This is for EME, which does not require much
movement. Since it is so long, it won't be variable in the elevation
angle anyway. But, I hope to move one wire a bit so I can steer the
beam to the left or to the right a little (changing the angle of the
vee at the same time).


Hi Art,

Too many details lacking. 40 to 60 covers too much turf (beyond the
pun), you neglect how wide it will be at the ends too. I ran a couple
of tests to see the gain from 17 to 22dBi and the Z anywhere from 200
Ohms to 2000 Ohms. One thing to consider is the "law of diminishing
returns." The gain for a much smaller V is not much different from
the giant one (think 3dB for each doubling - as a shorthand, I am sure
this fails long before the 20, much less 50th, wavelength is reached).

73's
Richard Clark, KB7QHC



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On Wed, 07 Jul 2004 19:16:36 -0400, Albert wrote:

OK, thanks Richard.

How about this.

30 wavelengths per side, average conducting ground, 12 feet above
ground.

You still don't say how far apart the tips are.

What angle do I need for best forward gain, what will the takeoff
angle be at that angle?

The angles were pretty consistant at 5 degrees at 12 feet up. Gain
did not vary much either (5dB is a lot perhaps, but then there are so
many variables to consider).

If I can get the entire antenna up to 20 feet above ground, how much
better is the performance?

Height change up doesn't seem to be signficant.

Is there any way for me to do modelling myself without burdening you
or someone with a modeling program? I am not aware of free software
that will model rhombics or vee's.

Thanks,

Art

Hi Art,

With monster size antennas and a desire for accuracy, this drives the
model towards a lot of segments ($$$). You could use the free version
of EZNEC but it will blow up.

EME hmmm? Major lobe is 5 degrees off the horizon and 5 degrees wide.
Isn't there a problem with an antenna looking at the moon so close to
the horizon (ground temperature)? Also, the moon will only fill that
box for, what, 15 minutes?

You might find it simpler to build an array of dipoles such that they
were all looking up at the same box. Combining them may be a bitch
however, so I can see your desire for simplicity here.

73's
Richard Clark, KB7QHC

Art

Which brand of EME is this for? If for one of the new digital modes,
you don't need that much gain, as you may already know. If it's for CW,
having a ton of gain in one direction only gives you a very seldom
available and very short window.

You would be better off sticking up about 15dBd of rotatable gain and
making a horizon sched with W5UN. The 2m EME net is on 14.345 at 11AM
central saturdays and sundays. Net control is VE7BQH. Dave, W5UN, is
almost always on.

Dave also does the digi EME modes, and as a guess could probably work
you on something like 10dBd with 100W on one of those.

I know Ian is an EME're, so pipe in, Ian!

tom
K0TAR

Albert wrote:

OK, thanks Richard.

How about this.

30 wavelengths per side, average conducting ground, 12 feet above
ground.

What angle do I need for best forward gain, what will the takeoff
angle be at that angle?

If I can get the entire antenna up to 20 feet above ground, how much
better is the performance?

Is there any way for me to do modelling myself without burdening you
or someone with a modeling program? I am not aware of free software
that will model rhombics or vee's.

Thanks,

Art

ky1k at pivot dot net

PS: Some additional info. This is for EME, which does not require much
movement. Since it is so long, it won't be variable in the elevation
angle anyway. But, I hope to move one wire a bit so I can steer the
beam to the left or to the right a little (changing the angle of the
vee at the same time).

You still don't say how far apart the tips are.

What angle do I need for best forward gain, what will the takeoff
angle be at that angle?



Hi Richard,

When I asked 'what angle do I need for best forward gain', I was
hoping you could give me an idea what the optimum distance between the
tips was.

My hope was that longer legs might allow me to use smaller angles
although I have a big field to play in.

Thanks,

A


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On Wed, 07 Jul 2004 19:14:03 -0500, Tom Ring
wrote:

Art

Which brand of EME is this for? If for one of the new digital modes,
you don't need that much gain, as you may already know. If it's for CW,
having a ton of gain in one direction only gives you a very seldom
available and very short window.


Thanks for the eme advice.

But, I was asking about the gain of a vee beam because they are cheap
to build and easy to put back up when the weather takes them out. We
have major ice storms often here.

Which "Brand of eme' is irreleveant, gain is gain regardless of which
mode is used. Excess gain is never wasted, it either makes the QSO
faster or allows one to work smaller stations or allows SSB instead of
CW/digital modes.

If the angle of the V makes little difference to the actaul gain, it
can be steered by moving either leg, which is also easy to do.

Although I can only work on my moon rise or moon set, more gain is
always better. Since I can't elevate my yagi, a non elevatable vee
beam for transmit only might give higher gain. The yagi would be for
receive only.

Having different receive and transmit antennas has benefits as well,
such as no switching loss and no need to protect my gaasfet during
transmit.

Can you offer any suggestions regarding the original question?

Thanks

A


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Tom Ring wrote:
Art

Which brand of EME is this for? If for one of the new digital modes,
you don't need that much gain, as you may already know. If it's for
CW, having a ton of gain in one direction only gives you a very seldom
available and very short window.

You would be better off sticking up about 15dBd of rotatable gain and
making a horizon sched with W5UN. The 2m EME net is on 14.345 at 11AM
central saturdays and sundays. Net control is VE7BQH. Dave, W5UN, is
almost always on.

Dave also does the digi EME modes, and as a guess could probably work
you on something like 10dBd with 100W on one of those.

I know Ian is an EME're, so pipe in, Ian!


(Still sleepy and jet-lagged...)

By coincidence, I was involved in just that kind of thing in 1979(?)
when we made the very first 2m EME QSOs from G-land using amateur
antennas at both ends of the path. One of our group was a farmer's son,
so we were able to string a 600ft rhombic over a large field of pigs.

The quick answer about feeding a very long V-beam on 2m is to use a
"universal stub" - a half-wave open-wire stub with a shorting bar, and a
4:1 coax balun. Adjust the tapping points for the shorting bar and balun
to get a good impedance match, and away you go. The universal stub is
almost a lost art, but any *old* VHF handbook will show you how to make
one.

It's obviously much more convenient if you come down to ground level in
high-grade open-wire feeder - not the store-bought stuff, but home-made,
with close-spaced wires under tension and a minimum of insulators. You
can then do the matching at ground level.

30 wavelengths per leg should be long enough to eliminate any
termination requirements at the far end. Radiation "loss" from the
forward-travelling wave will automatically ensure that the rear lobe is
reduced. If you wish, you can terminate the far end of each leg with a
300R low-inductive resistor and two quarter-wave "radials" in a T
configuration... but you'll probably not notice the difference.

However, it's true that:
having a ton of gain in one direction only gives you a very seldom
available and very short window.

This is a major inconvenience - you get maybe 20 minutes total operating
time per day, on maybe 3-4 days per month maximum. And that's only if
the direction of the beam is perfectly optimized. You need to lay out
the antenna with an accuracy of about 1 degree maximum, so you'll need
to borrow some serious surveying equipment. Guess-and-compass methods
will not work, because even small azimuth errors could mean that you're
operating on completely the wrong DAY!

(As the one who did the calculations, I can still remember the feeling
of relief on verifying that the moon really did set in front of the
rhombic, and on the right day too.)

Also, these moonrise or moonset windows will occur at arbitrary times of
day or night. With absolutely no time to waste, you will be limited to
making skeds... and not many sked partners may want to share that
inconvenience with you.

Overall, I agree with Tom - a large fixed antenna was the right thing to
do 20+ years ago, but 2m EME is now in a very different place.

A smaller steerable beam will trade raw gain for a huge increase in EME
operating *time*, and with modern operating techniques, time is what you
need the most.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Hi Ian,

Thanks for your comments.

My hope was to use a v beam instead of a rhombic.

The V is relatively easy to steer, especially if the performance
doesn't change much if either leg is moved. Note that I hope to leave
one leg of the beam fixed and steer the az by varying the position
(and the V angle) of the other leg. Not sure how much changing the
angle of the V impacts the performance, which is why I asked for
someone with modeling software.

My hope was to exploit the high gain, simple construction (but poor
receive) of the V for transmit only and to use the modest sized FO
yagi for receive only.

If I can get 19 or 20 db from a big V beam, I can probably tollerate
the limited operating time as well, Q's with big guns should take only
minutes and medium sized stations should be workable in a 20 minute
window.

If the computer model predicts a usable gain, I'd like to try putting
one up.

Regards,

Art



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On Wed, 07 Jul 2004 19:16:36 -0400, Albert wrote:

|OK, thanks Richard.
|
|How about this.
|
|30 wavelengths per side, average conducting ground, 12 feet above
|ground.
|
|What angle do I need for best forward gain, what will the takeoff
|angle be at that angle?

Included angle from 12 to 16 degrees will give ~20 dBi at 6 deg
elevation.
|
|If I can get the entire antenna up to 20 feet above ground, how much
|better is the performance?

~ 2 dB more gain at 4 deg el.
|
|Is there any way for me to do modelling myself without burdening you
|or someone with a modeling program? I am not aware of free software
|that will model rhombics or vee's.

4nec2 is free, do a Google search

Multinec is cheap and very flexible.

|
|Thanks,
|
|Art
|
|ky1k at pivot dot net
|
|PS: Some additional info. This is for EME, which does not require much
|movement. Since it is so long, it won't be variable in the elevation
|angle anyway. But, I hope to move one wire a bit so I can steer the
|beam to the left or to the right a little (changing the angle of the
|vee at the same time).

Since I worked VK5MC to complete my 2-meter WAC and he used a slightly
steerable rhombic, I'm not going to say that this won't work, but I
must question why. If you're going to limit yourself to a few minutes
of moon time a month, why not just put up a long fixed Yagi and use it
for both transmit and receive. A forty-foot long Yagi will give the
same gain as the vee, with a *huge* improvement in the pattern and it
will have a much more tractable feedpoint impedance.

Wes