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Author Topic: 3:1 VHF Balun (70cm)  (Read 3956 times)
KG4SGP
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« on: June 26, 2008, 06:57:09 PM »

Hey every one!

Posted this in VHFUHF forum, sorry for the redundancy.

I just built a 24element 16.5 (5m) boom length yagi for the 70cm band and am using a folded dipole for the driven element. This gets the impedance in theory (and modeled) up to 151(modeled)ohms. I’m sure it will be a pretty close estimate so now I need to build a 3:1 balun for the antenna to get the impedance back down to 50 for the LMR-400 coax.

My question is this: How should I go about doing it? I've never built a balun for the VHF bands and wouldn't really know where to start.

Suggestions would be greatly appreciated! (I'm planning on using it on field day 0.0!!!)

-Jim - KG4SGP
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N3OX
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« Reply #1 on: June 26, 2008, 07:32:22 PM »

3:1 is a weird transformation ratio.  You can get quite close with a ferrite transformer with lots of windings.  You can do 150:54 ohms with "quintifilar" (5) windings on a toroid, tapped at 3 windings for the low impedance side,  but I'm not sure I'd try to do that at UHF.  

If you really need to do 150:50 ohms and get excellent SWR, I'd maybe recommend  building a twelfth wave transformer using 50 ohm cable and 150 ohm cable made from series connected 75 ohm cables (series connection means connect the shields together at both ends and use the inner conductors as balanced line)

http://www.tuc.nrao.edu/~demerson/twelfth/twelfth.htm

I'm sure there are lots of other options, but this one might use coax you already have on hand.

73,
Dan







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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K6AER
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« Reply #2 on: June 26, 2008, 08:50:08 PM »

I would build a 4:1 Coaxial balun and adjust the dipole feed impedance for 200 ohms. The problem with toriods is getting a good permeability at that frequency with out succumbing to higher than normal loss.
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WB2WIK
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« Reply #3 on: June 26, 2008, 09:10:30 PM »

A toroid at 450 MHz does just about nothing.

Air works as well as (actually better than) ferrite at that frequency.

I agree with Mike, adjust the DE for 200 Ohms and use a 4:1 coaxial balun.

Even better, unless you specifically need the extra bandwidth provided by a folded dipole DE, replace the DE altogether with a conventional split dipole driven element, and a T-match with a 1/4-wave coaxial balun.  

WB2WIK/6
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VK1OD
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« Reply #4 on: June 26, 2008, 09:34:20 PM »


Why would you design the antenna for an inconvenient feed point impedance?

If you look at successful designs, they don't just happen to have a feedpoint impedance that is practical, it is one of the design objectives and compromises are made to achieve a practical system.

Yes, there are a host of ways to achieve the matching you need... but the better way is to go back to the drawing board and play with the drive cell to get a convenient feed point Z while preserving most of the antenna performance.

You might need to study some practical matching schemes to get an understanding of what is practical. In your case, the nearest target is 200 ohms balanced with a 4:1 half wave balun.

BTW, most efforts at a gain or pattern optimised antenna wind up with dipole feed impedances below 30 ohms, so your 150/4 is relatively high. You have probably neither optimised gain or pattern, or feed point impedance.

Owen
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G8UBJ
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« Reply #5 on: June 27, 2008, 03:38:04 AM »

Reducing the distance between the driven element and 1st director will reduce the feed point impedance. You can achieve 50ohms this way but may compromise the front to back ratio or skew the pattern. 200 ohms is easier to achieve and then you just need a 4:1 balun (Which is easy to construct using coax)
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W8JI
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« Reply #6 on: June 27, 2008, 06:15:06 AM »

Reducing the spacing to DE1 will also INCREASE the feed impedance. That is a very common method of adjusting the impedance of the array, but of course DE1 is no longer really a DE.

I'm with everyone elese on this. Matching sections and transformers are a PITA on 432MHz because a wavelength is so small that any errors by connections are a major source of problems.

It would be far better to use an integrated matching system in the antenna, like a T match or hairpin, and do a simple 1/4 wave sleeve or grounded 1/4 wave transmission line balun, than it would be to use a folded dipole with some odd impedance.

If you add another parasitic element very close to the DE, you can raise the feedpoint impedance to anything you reasonably want with suitable element tuning and spacing. You can make it increase or decrease.

73 Tom

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W8JI
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« Reply #7 on: June 27, 2008, 06:18:24 AM »

By the way, a simple practical 1/4 wave line section of 75 ohms would give you a 1.3:1 SWR. What you gave up in SWR loss on the feeder would probably be LESS than then what you give up through fancy matching systems, although I'd still use a T match or a hairpin with a split element.
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W5DXP
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« Reply #8 on: June 27, 2008, 06:47:26 AM »

You might try 1/4WL of RG-62 or RG-133 with a Z0=93 ohms. If the feedpoint is 151 ohms, it will be transformed to 58 ohms which should be a close enough match to 50 ohms.
--
73, Cecil, w5dxp.com
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WB6BYU
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« Reply #9 on: June 27, 2008, 08:18:07 AM »

Or use a beta match:  a small shunt inductor across the
150 ohm feedpoint to raise the impedance to 200 ohms,
followed by a conventional 4 : 1 coaxial balun.
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W5DXP
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« Reply #10 on: June 27, 2008, 02:09:48 PM »

> WB6BYU wrote: Or use a beta match: a small shunt inductor across the 150 ohm feedpoint to raise the impedance to 200 ohms, ... <

Doesn't a shunt reactance lower the impedance?
--
73, Cecil, w5dxp.com
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
VK1OD
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« Reply #11 on: June 27, 2008, 03:10:59 PM »


Someone said: "Doesn't a shunt reactance lower the impedance?"

No, not necessarily.

Do the maths, explore resonance effects when the antenna is adjusted for a small series R and some -ve X, then shunt it with an inductor.

Try this example: You have a feedpoint R of 20 ohms and you shorten the DE to introduce a series X of -60. This has Y=0.005+j0.015. Now shunt it with a hairpin with susceptance -j0.15.

(The G value of 0.005 is no accident, I want an outcome of 200 ohms feedpoint Z.)

Using http://www.vk1od.net/tl/twllc.htm , I can see that a hairpin of 5mm conductor spaced 25mm centre to centre has a susceptance of -j0.15 at about 26mm length... so there are the approximate dimensions of a hairpin to raise a 20-j60 load to 200+j0 for a half wave balun feed from 50 ohm coax.

Basic concepts of resonant circuits, but using the antenna feedpoint and a short s/c TL stub... too easy?

Owen
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WB6BYU
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« Reply #12 on: June 27, 2008, 04:10:01 PM »

As in the example Owen showed above, a shunt reactance
raises the feedpoint impedance when the driven element
length is adjusted accordingly to bring it back to resonance.


In this specific case, if the driven element is movable
I'd suggest just feeding it with a 4 : 1 coax balun and
sliding the driven element along the boom to improve the
SWR as needed.  (Of course modeling the antenna first to
see how this affects the pattern, gain and bandwidth is
a good idea.)  This avoids the need to fuss with shunt
elements, stubs, etc.

An alternate approach would be to choose two diameters of
copper or brass tubing to make a quarter wavelength line
with a characteristic impedance of 86.6 ohms.
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W5DXP
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« Reply #13 on: June 28, 2008, 12:26:53 PM »

> VK1OD wrote: Someone said: "Doesn't a shunt reactance lower the impedance?" No, not necessarily.

Would you please show how adding a shunt reactance to 150+j0 ohms can result in raising the impedance?
--
73, Cecil, w5dxp.com
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WB6BYU
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Posts: 13281




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« Reply #14 on: June 30, 2008, 10:36:05 AM »

You also have to change the antenna length to add some
reactance at the feedpoint.
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