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Author Topic: HomeBrew 12:1 Balun for Sloping V Beam Antenna  (Read 9154 times)
KC8FZL
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« on: August 17, 2009, 07:14:52 PM »

Need schematic or diagram to build 12:1 Balun for sloping V Beam Antenna from someone who has done it and made it work.  Will be using this system on Air Force MARS.
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G3RZP
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« Reply #1 on: August 18, 2009, 09:36:10 AM »

The IEE 'Convention on HF Communications' Proceedings from 1963 has a good description (P370 et seq) of one, although it might be a bit big for you, being meant for 40kW......it would lose between 400 and 1200 watts as heat with 40kW in.
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KC8FZL
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« Reply #2 on: August 18, 2009, 07:25:58 PM »

Thanks for the info.  Unfortunately 40 kW is above and beyond.  Will check out this reference first chance I get just for fun!
KC8FZL
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KB1LKR
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« Reply #3 on: August 18, 2009, 08:10:49 PM »

You might be able to scale it down to say  400W to 4kW -- reasonable values for amateur (or MARS) use.
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G3RZP
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« Reply #4 on: August 19, 2009, 12:15:47 AM »

Basically, what they did was to stack a load of ferrite torids on top of each other, with 'fins' in between for the cooling. Two of these stacks sit side by side, with compensating capacitors and inductors. From memory, the ferrite toroids were about 5 inches in diameter: the fins were about 12 by 18 inches. The whole thing could be used outdoors, and again from memory (it's about 45 years since I saw one) weighed about 60 or 70 pounds.

These were basically a mixture of a conventional autotransformer with a balun winding on the lines of a transmission line transformer. They were a pretty succesful design. One question to answer is what sort of SWR is there at the 600 ohm level, and what can be withstood at the 50 ohm level.
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WB6BYU
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« Reply #5 on: August 19, 2009, 08:45:20 AM »

As a start you would wind 7 bifiliar turns on a suitable
ferrite core, connected like a standard 4 : 1 voltage
balun.  The two windings in series go to the 600 ohms
balanced.  The coax shield connects to the center tap
between the two windings.  The coax center conductor taps
one of the windings 4 turns from the center tap.  This
gives a ratio of 14*14 / 4 * 4 = 12.25.

If you need more inductance you can use 12 bifiliar turns
with the coax tapped 7 turns from the center, giving a
ratio of 11.76.

You can go to a trifiliar winding, but that makes it more
difficult to connect the coax shield at the center (which
is required to get good balance, unless you add a current
balun on the coax side.)  7 trifiliar turns with the
coax tapped across 6 turns of the center winding would
also give a ratio of 12.25 : 1.

It is difficult to get good coupling among the wires
with more than 3 in a bundle.

If you have a long enough feedline (a few wavelengths
perhaps), Laporte's "Radio Antenna Engineering" book
(available online) shows a wideband transformer using
a tapered transmission line that could go from, say,
600 ohms to 200 ohms, allowing the use of a conventional
4 : 1 balun.

Whether these designs work for you will depend on how
constant your 600 ohm impedance is and the allowable SWR
range on the 50 ohm side.  Many of the nominal 600 ohm
antenna designs actually show a fair variation in SWR
over the operating range, and you may find that no fixed
balun will provide an adequate match without some additional
matching.  In that case you might as well run the 600
ohm line to the shack and use a balanced tuner there.
If you are using several fixed frequency bands it is
easy enough to design a band-switched tuner with fixed
components to match the specific impedances encountered
(perhaps with one variable capacitor for fine tuning.)
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KC8FZL
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« Reply #6 on: August 20, 2009, 01:14:19 PM »

Might you have a drawing or schematic for this?  Thanks.
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K0IZ
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« Reply #7 on: August 21, 2009, 02:50:42 PM »

Nellie, why do you want a 12:1 balun?  I use V beams and a V beam of 2 wavelengths or more has about a 110 to 120 ohm impedance.  So I use a 2.25:1 union plus a 1:1 balun, and get virtually 1:1 SWR.   John.
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K0IZ
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« Reply #8 on: August 21, 2009, 03:04:21 PM »

As an adenndum to my post above, my V beams are resonant in the bands (See QST June 2005), so the 110-120 ohm impedance is the resonant impedance (each leg is odd quarter wavelenth).  None resonant V beam would vary upward from the 110 - 120 ohms.
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KC8FZL
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« Reply #9 on: August 21, 2009, 05:06:00 PM »

Actually, I posted this for a friend, N8PYR/AFA5RK.  I will put your question to him and get back to you asap.  I do appreciate your input.  Please call me Jean.  Thanks.
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KC8FZL
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« Reply #10 on: August 21, 2009, 06:20:57 PM »

My friend tells me his info re 12:1 is from a Department of Defense Electromagnetic Capability Analysis Center, Annapolis, MD, Field Antenna Handbook, June 1984, pages 49-51.  It is a sloping V beam antenna terminated with 300 ohms with 3,000 resistors that should handle half of the transmitter's power output.  It uses 600 ohm open wire feed line with a 12:1 balun.

I hope this makes more sense to you than it does to me.

If there is any way he can reach you to talk since he does not have a computer, it would be nice.  If you would like his phone number, I can give you that if you say the word.  He could call you on cellphone without cost if he had your number.  Again, just say the word.  He needs all the help he can get in this venture!
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W8JI
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« Reply #11 on: August 22, 2009, 12:24:23 AM »

You know of course the gain of a V Beam is actually pretty low compared to other antennas. You are actually going to be lucky if it exceeds the gain of a good dipole.

The theoretical gain was always much higher than what the antenna really does in real life.

You can see some models of Rhombics on my web site:

http://www.w8ji.com/rhombic_antennas.htm

A properly designed V beam has at least 3dB less gain than the Rhombic.

The impedances are reasonable for a terminated V. You can expect about 500-1000 ohms of feed impedance depending on frequency, height, and wire size. It actually takes a "cage" of wires simulating a thick conductor to get down to 300 ohms on a leg. Normally a single #10 wire would be about 500-600 ohms, so a 300 ohm resistor would slightly mis-terminate the antenna.

A broadband 12:1 higher power balun with low SWR is not an easy task. 4:1 is easy, 9:1 significantly more difficult, and 12:1 is very difficult.

Personally I think he would be better off with a tuner.

I understand there is a fascination with V Beams and Rhombics, but my honest recommendation is he stay away from them. They have about the lowest gain for sin X/X pattern width of any antenna available, and by far the lowest gain per acre of room. This is because they have poor efficiency and many small lobes that waste the power does not turn directly to heat.

They are a cheap and easy way to build a very high power directional broadband antenna, but that is about the only advantage.

I have several 200-300 foot towers here and room for V beams and Rhombics, but after researching them (General Dynamics measured data) and modeling them I could never find a V beam design that significantly beats a dipole, nor a Rhombic that significantly beats a 3 element Yagi. In the 1970's I had a sloped V beam (with two wavelength legs) for 160 meters on a 355 foot tall broadcast tower with a swampy area below the antenna, and I actually found a 130 foot tall vertical at my house a few miles away often tied it for DX. It was a large project but never worked out for any real gain, although it received OK. After a few weeks I went back to a droopy dipole on that tall tower.

I don't want to be a wet blanket, but if your friend is looking for gain he probably wants to look at some other system. If he needs a simple low gain antenna with wide bandwidth that can handle very high power, then the V would be a good solution. I would NOT expect a low SWR however with 300 ohm terminations on each wire, because the surge impedance of a small conductor high above ground is well above 450 ohms. It would have noticeable ripple in SWR with 300 ohm terminations on each wire and a 600 ohm feed. I would expect a 2:1 or higher on SWR peaks as frequency is varied. Think of it this way....a #14 wire six feet above ground is about 500 ohms or so impedance. Why would a higher wire only be 300?
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K0IZ
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« Reply #12 on: August 22, 2009, 05:25:58 PM »

I agree with Tom that a V beam has some 3 db less gain than a rhombic - which is appropriate since a V beam is essentially one half of a rhombic.  A rhombic can have loads of gain in its desired direction, which could be either bi-directional (not terminated) or directional (terminated).  As so V beams.  

I use a four leg V beam, resonant on five bands using coaxial traps.  Can switch different pairs for directivity.  EZNEC simulations of really good gain have been born out with operating experience.  I have frequently been told by DX that I am the loudest signal on the band ( 20 meters), hardly what would be expected with dipole-type results.

The V beam is an honorable, historic, antenna.  It is generally built as a non-resonant antenna, and fed with open wire line.  Assuming non-resonsnce I would be inclined towards a 4:1 balun since remaining SWR on the open wire feedline would be of no practical consequence.  I recommend looking at one of Jerry Sevick's books on the subject for good design info.
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WB6BYU
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« Reply #13 on: August 22, 2009, 07:53:31 PM »

Terminated Vee-beams can provide a reasonably directional
pattern over an octave or more with a relatively tame
input impedance.  They aren't ideal, by any means, but
if you want a little bit of gain and directivity from a
simple wire structure over a range of frequencies (rather
than specific narrow bands, as is the case for amateur
use) then it may not be a bad choice.

We've designed vee-beams for the low-VHF range for military
tactical communications where F/B was desired to reduce
the chance of intercept (though they certainly aren't
perfect in this regard) and I worked with a large vee
beam with 5 sets of wires (all of different lengths and
sloping at different angles) that was set up for MARS
communications from California to Viet Nam, which had a
better signal into Asia than any of the yagis that were
installed at the station.  (That's not to say that a
bigger or higher yagi might not have been better.)  I've
also set up several vee beams for Field Day.


I'm afraid that I don't have drawings available for
the balun designs I suggested - I just invented them
for the occasion rather than reporting an existing
design.  If you find the drawing for a conventional
4 : 1 voltage balun you will see it consists of a
bifiliar winding (two wires wound together as one but
connected individually.)  The two wires are connected
in series across the antenna, and the coax shield is
connected to the common connection to them.  The coax
center conductor is connected to one end or the other
(it doesn't matter.)  Since the coax is connected
across one winding and the antenna across two in series,
the turns ratio is 2 : 1, so the impedance ratio is the
square of that, or 4 : 1 (balanced to coax.)  This
process can be extended using more wires in the bundle,
to make a 9 : 1 (3 wires) or 16 : 1 (4 wires).  But
you can't get a 12 : 1 with integer windings since it
requires a turns ratio of the square root of 12, or
about 3.46.  (If you could wind 7 wires together and
connect the coax across 2 of them, that would come
pretty close with a turns ration of 3.5 : 1, but there
are significant practical difficulties with using any
more than 3 wires.)


So the solution I suggested taps the windings somewhat
short of the full number of turns across which the
antenna is connected.  In theory it is simply a matter
of making sure the coax shield is connected to the
center tap of the antenna winding (that is what makes
it a balun) and getting the right turns ratio close to
3.46 or so.  So with 7 bifiliar turns (14 total) across
the antenna you need 4 across the coax to get 3.5 : 1
turns ratio.  (It isn't practical to tap a toroid
transformer at fractional turns, but I don't think you
will see a problem if the ratio actually is 12.25 : 1.)

This isn't as simple to wind as it might seem:  probably
the easiest way is to twist a kink in one wire so you
have a point to solder the coax center conductor, then
lay it in parallel with the second wire, twist them
together (or not), and wind the pair onto the core
starting at the tap point and running in in each direction
around the core to get the proper number of turns.  That
probably is easier than trying to tap one of wires
after it has been wound on the core.

In the real world, the practical details make a huge
difference, and will depend on the choice of ferrite
material, the frequency range to be covered, the effect
of stray capacitance and inductance, etc.  The 40kW
balun that was mentioned previously included compensating
inductors and capacitors to improve the match over the
desired frequency band:  this may or may not be needed
in your case depending on the range of frequencies that
you want to cover, the actual impedance of the antenna,
and the allowable SWR at the transmitter.

This is why it may actually be easier to build a
balanced antenna tuner if you are only concerned with
specific segments of the frequency range, as it can be
adjusted to accommodate for all these variables, as
well as less-than-perfect impedance at the antenna.

My email is in my eHam profile - feel free to drop me
a note.
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W8JI
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« Reply #14 on: August 23, 2009, 07:16:09 AM »

An optimum 4 wavelength per leg Rhombic has about the same gain as a 4 or 5 element Yagi.  This is why VOA and other moved away from Rhombics, and why commercial point-to-point stations all started switching to much higher gain HRS curtains or log periodics.

A V beam is even less gain per physical area. You will have a difficult time finding an HF V beam that makes more gain than a dipole. Try to model one and see what you actually find. You'll be amazed.

The only real advantage is they are easy to build, and they can handle a ton of power over a wide bandwidth.

Tom
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