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Author Topic: Antenna Tuner and Balun questions  (Read 2461 times)
KK4CPH
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« on: January 12, 2013, 12:55:28 PM »

Been researching this and I'm finding different answers about which to use.  Here's my situation:  I have a 31' Eagle One vertical in the back yard 150' from my house.  It came with a 4:1 Balun and I was using my Kenwood TS-440S with auto tuner. BUT now I have an ICOM-706mkII and it doesn't have the auto tuner.  I'll have to get one (looking at the LDG IT-100) So while doing some reading I've found some say there's no need for a Balun with an antenna tuner.  Others say you need it.  Also read the tuner should be in the shack and others say it should be at the base of the vertical.  Huh  Would welcome some clarification on this. 

Eric
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N3JBH
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« Reply #1 on: January 12, 2013, 01:08:17 PM »

Well with 150 run of feed line i definitely say a tuner at the antenna be the best bet. I also suggest you lay a bunch of radials on the ground as well if it is ground mounted, If not you want some other advice. With the tuner mounted at the antenna i see no reason for a balun. But i really do not think that antenna will be much of a performer below 40 meters either.   
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WB6BYU
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« Reply #2 on: January 12, 2013, 07:04:30 PM »

First, you don't really want a balun at the antenna.  If you put any sort of matching
device there it should be an un-un, that is, both sides are unbalanced.  Using the
wrong sort of balun here actually can cause the outside of the coax to radiate
more than it would otherwise, and if that coax runs along the ground it will dissipate
power in the dirt.

The reason for using a 4 : 1 step-up transformer (un-un) at the antenna is to
lower the SWR on the feedline on some bands, thereby reducing losses in the
coax.  This is more important with longer feedlines and/or those using smaller
sizes of coax, where the losses will be higher.

If you are going to use an autotuner, the best place for it is at the base of the
antenna, because then the coax is operating at a low SWR on all bands.  That's
more efficient, thought the amount of difference will vary from band to band.
A tuner (automatic or manual) in the shack will match the impedance so the
radio can deliver full power into the coax, but that doesn't mean that all the
power reaches the antenna at the other end.
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KK4CPH
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« Reply #3 on: January 12, 2013, 09:05:01 PM »

Thank you both for the responses.  I wonder why I was told I needed the balun when I bought the antenna?
I did tune up on 10m, 20m, and 40m and noticed that my SWR readings were right around 1.5 (both on the Icom and a separate SWR meter) with the balun at the antenna.  Makes me wonder if I really need the antenna tuner.
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AC2EU
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« Reply #4 on: January 12, 2013, 09:48:37 PM »

Thank you both for the responses.  I wonder why I was told I needed the balun when I bought the antenna?
I did tune up on 10m, 20m, and 40m and noticed that my SWR readings were right around 1.5 (both on the Icom and a separate SWR meter) with the balun at the antenna.  Makes me wonder if I really need the antenna tuner.

I would suspect that the impedance should be all over the map depending what band you are on. A balun would be of little use for an "multiband" antenna such as yours. The mfr suggests a remote tuner at the antenna due to large impedance variations. This way there will be a good match to the coax thus reducing loses there. 
Believe me, you do need a tuner!
The physics of the situation makes me wonder how it is possible to have a 1.5 swr without a tuner on all of those bands. There may actually be something wrong...

As to why he sold you a balun? That's easy to figure out...he made more money!!  Grin
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G3RZP
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« Reply #5 on: January 12, 2013, 10:11:12 PM »

Plus losses in the balun help reduce the SWR....but do nothing helpful for radiated power.
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VA7CPC
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« Reply #6 on: January 12, 2013, 10:36:00 PM »

150' of coax is a substantial run.

If there are significant losses in the coax, the SWR at the rig will tend toward 1:1 _regardless of what's at the far end of the coax_.   If it's wet, or buried (and damp), the losses will be increased over dry coax.

So the next question:

. . . What kind of coax?

. . . What are its losses (db / 100') at the frequencies you're using?

Multiply the losses per 100' by 1.5 to get the loss (in dB) for 150'.

If the answer is much over 3 dB, your answer to "why is the SWR around 1.5:1?" is:

. . . most of your power is dissipated in the coax, not radiated by the antenna.

.              Charles
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W5DXP
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« Reply #7 on: January 13, 2013, 05:50:19 AM »

If the (coax loss) is much over 3 dB, your answer to "why is the SWR around 1.5:1?" is:. . . most of your power is dissipated in the coax, not radiated by the antenna.

And an autotuner at the base of the vertical eliminates the coax losses due to excess SWR. Of course, there are tuner losses but if the vertical is at least 3/16 wavelength long on the lowest frequency of operation, tuner losses will probably be acceptable to most hams and well worth the expense for a 150 ft run of coax. Here's the vertical I ran with an SG-230 autotuner at my last QTH:

http://www.w5dxp.com/vert4010.htm
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KK4CPH
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« Reply #8 on: January 13, 2013, 11:52:27 AM »

I'm using RG8X coax.  The antenna is 150' from the house because I don't have much of a choice.  Don't need to drill holes in my new roof and stick a 31' lightning rod on it.   Grin   I live on a steep hill.  Go down about a foot into the ground and it's solid rock.  (they had FUN putting in my septic tank!)  Drain field in the back yard, power lines in the front, and not much of a side yard.  So it was either put the antenna next to the house or near the end of the property.  My only other option might be something like the Chameleon EMCOMM II, maybe off the roof edge to a tree(?). Then I would only need about 25' coax. 


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KE6EE
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« Reply #9 on: January 13, 2013, 12:25:24 PM »

As others have pointed out the 4:1 gizmo at the antenna base is an unun not a balun. It's essentially an impedance transformer which likely reduces the swr the coax sees at the base of the antenna on most of the bands you use. Without the transformer your feedline losses will likely be larger.

With a relatively long run of not-so-efficient coax you might think of another feed system. How about a 1:1 balun at the antenna base and then a run of open wire feeder to a 4:1 balun to a regular tuner or a direct connection to a balanced line tuner? How about very-low-loss hardline (expensive new but can be very cheap surplus) run from antenna unun to tuner?

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KK4CPH
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« Reply #10 on: January 13, 2013, 02:40:51 PM »

I don't mind starting over.  Live and learn!  No sense of torturing myself with an inefficient setup.
What coax should I get? RG-214? LMR-400?

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WB6BYU
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« Reply #11 on: January 13, 2013, 03:23:20 PM »

RG-213 is fine for 150' at HF, as long as the SWR is reasonably low.
(RG-214 won't be any better - it just has a second layer of shielding, but
the losses are the same.)

The problem is that the SWR will be high on several bands, and that will cause
higher coax losses.  While LMR-400 does have lower loss than RG-213, the numbers
for 150' of matched coax are 0.7dB and 1.1dB on 20m.  But if the load impedance
is 300-j700 ohms (a number that I pulled out of the air as an example) the respective
losses are 6.1dB and 7.7dB:  that's more of a difference, but even with the better
feedline you are losing 3/4 of your power in the feedline.

That's why matching the antenna at the feedpoint, rather than in the shack, is
a better approach.  Or at least some sort of preliminary matching (of which the 4 : 1
un-un makes a small start, but a homebrew switched coil/capacitor would be better)
will lower the SWR, even if you still have to do final matching at the rig.
« Last Edit: January 13, 2013, 03:28:58 PM by WB6BYU » Logged
KK4CPH
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Posts: 154




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« Reply #12 on: January 13, 2013, 04:40:21 PM »

I plan on getting the tuner, but if the RG-8X that I currently have is inefficient then I'll replace with it a better coax.
Could you explain the homebrew switched coil/capacitor or a link to it?  Thanks.
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WB6BYU
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« Reply #13 on: January 14, 2013, 08:55:05 AM »

You can use VK1OD's online Transmission Line Loss Calculator to check the loss of
different types of coax, and a lot more:

http://vk1od.net/calc/tl/tllc.php

RG-8x has 1.6dB loss in 150 feet when matched, so 2/3 of your power gets radiated.
That's acceptable for a lot of purposes, but the losses will be even higher than
RG-213 at a high SWR.  But still, the losses will be several dB higher using low-loss
LMR-400 matched at the shack than RG-8X matched at the antenna, so that gives
you an idea of where you'll get the best return for your investment.

(You'll find RG-8X listed as Belden 9258 in VK1OD's calculator, since it is NOT a
MIL-STD designator and there are no standards for it across manufacturers.)

Here is VK1OD's evaluation of feeding options:

http://vk1od.net/antenna/multibandunloadedvertical/index.htm


Any impedance can be matched using an "L" network using one coil and one
capacitor (though some loads may match easier using two capacitors or
two coils.)  Often a single series or shunt coil or capacitor can get the SWR
low enough (perhaps below 5 : 1 or so) to keep losses reasonable on the
coax.  The required value will be different on each band, so some sort
of switching is required.

When I put up an antenna at home that I expect to be using for an extended
period of time, I build a switched tuner for it.  I have a chassis with a big
switch and a chunk of coil stock mounted on it.  By experiment I find what
value of coil and capacitor are needed to match the antenna on each band,
then wire up the switch to select the required coil tap and fixed capacitor.
Then when I want to change bands I just turn the switch - instead of having
to adjust the tuner controls, they are preset to the right values.  A similar
approach can be used with relays or a remote switch of some sort (or, if
you don't change bands often, a manual switch at the base of the antenna)
that gives you the function of matching at the antenna feedpoint for less
cost than a wide-range auto-tuner.
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KK4CPH
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Posts: 154




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« Reply #14 on: January 14, 2013, 09:51:27 AM »

WB6BYU, thank you.  The link for the write up on the multi band vertical is great.  I was amazed at the amount of loss below 7MHz.

Eric
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