EMRFD Message Archive 3232

Message Date From Subject
3232 2009-06-21 18:04:48 Vojtěch Bubník balanced antenna with unbalanced tuner & balun?
Hi gang.

I am using Elecraft T1 automatic antenna tuner to tune 49ft dipole fed with ladder line. I placed 1:1 current balun between TRX and antenna tuner. The L network in the automatic tuner has reasonable low loss if feeding unbalanced load. This system tunes fine from 40m up.

Referencing "A Balanced Balanced Antenna Tuner" by Richard AG6K, he states that using balun on the antenna side of a tuner is bad and I understand his explanation. He also states that using unbalanced tuner creates inbalance in antenna currents into both dipole legs.

Frankly I do not understand how the whole antenna system with balanced dipole, balanced feeder, unbalanced ATU and 1:1 current balun balances itself and how well it does it. I suppose the balun will bounce the "bad" currents back until they are radiated either as RF energy or heat. OK, sum of RF radiated and heat radiated energy at each output leg of the balun will be the same. But probably the ratio of heat / RF will be different for both legs? How different it will be? What amount of losses may I expect if compared to a "balanced balanced" antenna tuner?

Thanks for the enlightment,
Vojtech AB2ZA
3234 2009-06-23 10:01:50 Stan Re: balanced antenna with unbalanced tuner & balun?
Vojtich the problem you will find anytime you touch an antenna or tuner everyone has a different opinion.

I do have a question for you to consider. You have a balanced antenna and feed line. Good start, dipole a little short but still a good start.

I would have put the 1:1 balun between the tuner and the feed line. The Elecraft T1 (i have never had one so might be wrong) I am sure is an L tuner and therefore unbalanced. So I would want it on the same side of the balanced balun as the transmitter which will also be unbalanced if it is a commerical product. Also I suspect if you read the specs on the tuner it only has to achieve a 2:1 match, at least the auto tuners I have used are only spec to that figure since they use fixed inductors and caps to obtain a match.

You did not say how long your feed line was. I would try and get it long enough that the antenna element and feed line was a multi- of 1/2 wave lengths if possible. I am from the resonant school on antennas and someone else may have a better idea.

I also use a Grid Dip meter to set my antenna up and do not use lossy tuners. Most good baluns have a very low loss but I do not use them if I can achieve a match without them.

You are on the right track you are reading the articles, the books and looking beyond the BS one gets on most antenna.

Stan ak0b
3235 2009-06-23 11:19:38 Sam Morgan Re: balanced antenna with unbalanced tuner & balun?
Stan wrote:

> I would have put the 1:1 balun between the tuner and the feed line. The
> Elecraft T1 (i have never had one so might be wrong) I am sure is an L
> tuner and therefore unbalanced. So I would want it on the same side of
> the balanced balun as the transmitter which will also be unbalanced if
> it is a commerical product. Also I suspect if you read the specs on the
> tuner it only has to achieve a 2:1 match, at least the auto tuners I
> have used are only spec to that figure since they use fixed inductors
> and caps to obtain a match.
>

FYI ... to quote Elecraft on the T1:
"The T1’s 7-inductor, 7-capacitor L-network provides a wide matching range, and
its re-tune time from memory is just 1 to 2 seconds. Equally important, the T1
always tries to achieve a 1.0:1 SWR—it doesn’t stop at 1.5:1 or 2.0:1 like some
tuners."

http://www.elecraft.com/T1/T1.htm



--
GB & 73
KA5OAI
Sam Morgan
3236 2009-06-23 16:57:03 Allison Parent Re: balanced antenna with unbalanced tuner & balun?
3237 2009-06-23 22:00:24 Ashhar Farhan Re: balanced antenna with unbalanced tuner & balun?
these days, i have been experimenting with various ways of putting up
an open line feeder and a doublet. someday i will be able to sit back
and write about it ... but right now, all i want to say is never trust
a rock to stay wedged under the 14 feet ladder. but onto the topic at
hand...

first off, anyone trying out "open feeder wire + ATU" should read the
article on the balance balanced line tuner thoroughly
(http://www.g4urh.co.uk/downloads/_downloads/balanced_tuner.pdf). it
dispelled many myths I had about ATUs. but to answer question on the
balun, this is my take:

1. baluns should always be at the equipment end. this is because, here
it will match a well defined impedance (50 ohms on one side and 50
ohms on the other side). baluns are designed to work well at certain
impedances. of course, one could design a balun for 1000 ohms. but the
inductance required would be different. it would probably require an
inductance with 4000 ohms reactance at the least. this is difficult to
achieve while maintaining broadband nature of the balun. so, to put it
simply, use a balun where the impedance is low and well defined. the
best place is near the equipment end.

2. for multiband antenna, it is best that you don't use an resonant
antenna. the resonance can result in either very low impedances (like
a half wave dipole with 75ohms impedance) or very high impedances (a
full wave dipoole). hence, a 7MHz dipole will exhibit a very high
impedance at 3.5MHz. listen to good ol' G5RV and use 51 feet
radiators instead.

3. the open wire feeder lines can be a fascinating project. i regret
not having dabbled in them earlier. building your own antenna feed
that easily out performs the best cable in the market at a fraction of
it's cost can be very enlightening. any spacing will do, any wire will
do, any spacers will work. just keep the line straight (as much as
possible) and hold it off the walls and roof by stand offs (i built
mine out of pvc pipe loose ends left over from my neighbor's
construction).

4. forget about buying an antenna tuner. what you need is that above
quoted article. i had to make to departures from the Richard's text.
first, i replaced the co-ax balun with a toroidal balun. second, i
didn't have the rolling inductors. so i wound mine on 2 inch PVC pipes
and used alligator clips to tap into the inductors.

- farhan

On Wed, Jun 24, 2009 at 5:26 AM, Allis
3240 2009-06-27 03:58:55 Vojtech Bubnik Re: balanced antenna with unbalanced tuner & balun?
Hi Stan.

> Vojtich the problem you will find anytime you touch an antenna or tuner everyone has a different opinion.

The biggest problem of antennas and feeders is that their performance is based on beliefs, not measurements. And to separate the BS from truth, I need to learn the theory and get feeling for the proportions. Academics often lack the latter, I currently lack both.

> I would have put the 1:1 balun between the tuner and the feed line.

Kevin Schmidt, W9CF cites Roy Lewallen, W7EL at page 8 of following article, and states, that the performance of balun before and after antenna is essentially the same.
http://fermi.la.asu.edu/w9cf/articles/balun.pdf

> The Elecraft T1 (i have never had one so might be wrong) I am sure is an L tuner and therefore unbalanced.

Yes.

> So I would want it on the same side of the balanced balun as the transmitter which will also be unbalanced if it is a commerical product.

The transceiver is ATS-3b, it's transmitter is single ended.

> Also I suspect if you read the specs on the tuner it only has to achieve a 2:1 match, at least the auto tuners I have used are only spec to that figure since they use fixed inductors and caps to obtain a match.

It mostly achieves better than 1:1.2 with my non resonant dipole.

> You did not say how long your feed line was. I would try and get it long enough that the antenna element and feed line was a multi- of 1/2 wave lengths if possible. I am from the resonant school on antennas and someone else may have a better idea.

Resonant system is a bad idea for multiband dipole. For even multiples of lowest band it will pose very high impedance to the balun and antenna tuner.

> I also use a Grid Dip meter to set my antenna up and do not use lossy tuners. Most good baluns have a very low loss but I do not use them if I can achieve a match without them.

That is a good rule of thumb for single band dipole (or odd multiple bands like 40m and 15m), not for multiband dipole like G5RV. Sure, I would do it if I had large enough property.

73, Vojtech AB2ZA
3241 2009-06-27 06:07:44 Vojtech Bubnik Re: balanced antenna with unbalanced tuner & balun?
Hi Farhan.

> first off, anyone trying out "open feeder wire + ATU" should read the
> article on the balance balanced line tuner thoroughly
> (http://www.g4urh.co.uk/downloads/_downloads/balanced_tuner.pdf).

I did. And I understand that the combination of balanced L matching network and 1:1 balun between transceiver and matching network is ideal. Now many commercial products recommend using 1:1 or 1:4 balun with their automatic antenna tuners. I simply would like to understand, how such a balun will be effective to balance the antenna and what would be the losses. And I was not able to find any calculations or measurements of balun losses if used with unbalanced tuner. I remember I saw only one evaluation of the balun between ATU and balanced feeder in QST archive and the losses were high. But this data is anecdotal as it evaluates only single balun design.

The automatic remote tuner is very attractive to those of us, who may one day end up in a condo with the only possibility to string a dipole on the roof of multiple story building. Access to a roof of such building is restricted also and feeding it with coax and using remote auto tuner is a simple robust multiband solution. And frankly not may of us have time to devise to build remote balanced auto tuner. If using switching elements, just buying the toroids and relays will be about as expensive as buying a factory made automatic L matching network.

> 1. baluns should always be at the equipment end. this is because, here
> it will match a well defined impedance (50 ohms on one side and 50
> ohms on the other side).

Based on Roy Lewallen findings (cited by Kevin Schmidt, W9CF in http://fermi.la.asu.edu/w9cf/articles/balun.pdf) the performance of the balun before and after the tuner is essentially the same if the tuner is unbalanced. My naive feeling for it is if you transform balanced highly reactive antenna by unbalanced tuner, the result will not be balanced against ground and the balun will have to cope with the same reactive load as if it was placed on the other side of the tuner.

> 4. forget about buying an antenna tuner. what you need is that above
> quoted article. i had to make to departures from the Richard's text.
> first, i replaced the co-ax balun with a toroidal balun. second, i
> didn't have the rolling inductors. so i wound mine on 2 inch PVC pipes
> and used alligator clips to tap into the inductors.

One day I will try it. But now I would like to understand baluns.

BTW, here are the measurement of my dipole with the VNA. What strikes me is how different the values are with and without the balun. I expected some differences, but I did not expect them to be so huge. Also the VNA is only reasonably precise around SWR 1:4.
http://www.kufr.cz/~ok1iak/HAM/Antenna/Antenna%20-%20Prospect%2027.pdf


I will try to learn as much as possible and then do some measurements. I will try to understand following article and then to build an antenna model from lumped elements and somehow measure the losses.
http://fermi.la.asu.edu/w9cf/articles/balun.pdf

I have a homebrew mini VNA designed by iw3hev. It could measure reflections and loss (one or two port measurements). Mini VNA two ports share common ground. Any hint on how to do model the antenna with the balun and measure balance and losses with my two port VNA? I hope it does not sound trivial, I consider myself to be a beginner in RF.

73, Vojtech AB2ZA
3242 2009-06-27 07:12:02 James Duffey Re: balanced antenna with unbalanced tuner & balun?
Vojtech - You wrote (in part):

"Resonant system is a bad idea for multiband dipole. For even
multiples of lowest band it will pose very high impedance to the balun
and antenna tuner."

This is not as bad as it seems. Most antenna tuners are more efficient
with high impedance loads than they are with low impedance loads. The
Q of the components have less of an impact on the efficiency with high
impedance loads than they do at low impedances.

Baluns: choke, current-mode or otherwise, are usually not operating
at their design impedance in most antenna tuner applications. And if
one uses the once ubiquitous 4:1 balun that puts the high impedance
load into a more favorable one for the antenna tuner, while for a very
low impedance load it will put the antenna tuner into a less favorable
operating condition.

There are some combinations of antenna length plus feeder lengths that
work better with tuners than others. One tries for a modest impedance
on all bands, but this can be difficult with the physical constraints
most hams have. - Duffey
--
KK6MC
James Duffey
Cedar Crest NM