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 Balancing an HF Doublet 
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Post Balancing an HF Doublet
My garden is too small to accommodate a straight 132 foot doublet so I had to
make it a "Z" shape to make it fit.
Due to surrounding buildings and trees I suspected that balance of the two legs
would be difficult to achieve and measure.
I therefore built two simple RF ammeters using ferrite toriods to compare
the leg currents. I installed the RF ammeters on the output of my home
made voltage balun, one per leg.
Initially I calibrated the RF ammeters using a 200 ohm resistor connected across
the output of the balun to ensure both meters read the same with a perfectly
balanced load.
When driving my doublet, the RF ammeters did show unequal leg currents which
varied between bands.

I progressively raised up one leg of the doublet until balance was achieved
on all HF bands.
A simpler way to achieve balance is to buy a house with a much bigger
garden, but that would be expensive !


Fri Jun 26, 2015 8:58 pm
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 Balancing an HF Doublet 
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Post Re: Balancing an HF Doublet
Do you realise that measuring identical currents in the two legs using that method is not a guarantee of balance?

Steve G3TXQ


Fri Jun 26, 2015 9:09 pm
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 Balancing an HF Doublet 
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Post Re: Balancing an HF Doublet
No I didn't realise that. OK so how do you measure balance?
I always thought that equal currents down both legs in a balanced feed
indicated balance.
I have no RF in my shack which I assumed was an indicator
of good balance.
I have checked along the ladder line for RF radiation with a field
strength meter which shows no radiation from the feeder.
I understand how the AC signal on each leg of the balanced feeder
has to equal in amplitude but 180 degrees out of phase to prevent
radiation from the feeder.
Without equal leg currents I assume the feeder would radiate.
Have I got this completely wrong?


Sat Jun 27, 2015 9:24 pm
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 Balancing an HF Doublet 
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Post Re: Balancing an HF Doublet
For complete balance, the leg currents must be of equal amplitude and 180 degrees out of phase. If you have equal leg current amplitudes but the relative phase is not 180 degrees, you have an imbalance and common-mode current flowing.

For example, if the current in one leg is 1A at 0 degrees and the current in the other leg is 1A at 150 degrees, you have 0.96A of Differential-Mode current and 0.26A of Common-Mode current.

Your arrangement measures the amplitude of the leg currents but tells you nothing about their relative phase; so simply getting equal meter readings is not sufficient to guarantee balance.

There are two easy methods to measure the balance properly:

1) Pass both legs through a single ferrite core and wind a multi-turn current pick-up coil on the core; if the coil picks up no signal the line is balanced.

2) Modify your present system so that the two separate leg current sensors are connected in series before rectifying with a single detector diode; zero output from the diode then indicates amplitude and phase balance. If that's not clear I'll sketch a schematic.

I'm not suggesting that you do have a CM problem - simply pointing out that you're measurement system can't tell you whether the line is balanced or not.

Steve G3TXQ


Sat Jun 27, 2015 10:00 pm
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 Balancing an HF Doublet 
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Post Re: Balancing an HF Doublet
Thanks Steve, that's brilliant.
I have a big chunky micrometals toroid here so I can
easily carry out the test as you suggest.


Phil.


Sun Jun 28, 2015 10:22 am
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 Balancing an HF Doublet 
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Post Re: Balancing an HF Doublet
VK5BR has a design based on connecting the two separate current sensors in series. Take a look at Figure 3 here:
http://users.tpg.com.au/users/ldbutler/ ... _Meter.htm

What he calls "longitudinal" current is the same thing as Common-Mode current. If you follow his schematic, you can see with the switch in the "longitudinal" position the two separate current sensors T1 & T2 are essentially connected in series.

Steve G3TXQ


Sun Jun 28, 2015 11:02 am
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 Balancing an HF Doublet 
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Post Re: Balancing an HF Doublet
Hi Steve, I have made some measurements using a large
toriod with both legs of the balanced feeder going through it.
I wound 25 turns onto the toroid.
There is obviously some imbalance because I do get a reading on the
meter.
With 100W on 40M I read 2 volts DC. I suppose that's 2.2 volts taking
into account the voltage drop across the germanium detector diode.
I now need to decide if the reading represents a reasonable or a poor balance.


Sun Jun 28, 2015 12:21 pm
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 Balancing an HF Doublet 
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Post Re: Balancing an HF Doublet
I have taken 15 turns off of the toroid to leave 10 turns
and connected an AVO set to milliamps.
The readings vary widely depending where along the feeder I place the toroid.
I assume that's due to high SWR on the feeder.
However on 18MHz the SWR is low so I don't need to use the ATU.
On 18MHZ the current I read is 10mA with 100W out.
Taking into account the 10 to 1 turns ratio of the toroid I assume
that's 100mA of longitudinal current flowing in the balanced feeder.
I guess I now need to raise or lower one leg of the doublet until I
read zero current for good balance.


Sun Jun 28, 2015 4:24 pm
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 Balancing an HF Doublet 
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Post Re: Balancing an HF Doublet
I'm not too sure of your exact measurement set-up!

The sense coil on the toroid needs to be terminated in a small value resistor - say 22 Ohms to generate an RF voltage. Rectify that voltage with a detector diode, and measure the resultant DC. The common mode current in the line would then be given by:

Icm = (Vdc+0.5) x N / 22

where N is the number of turns.

Don't confuse SWR with imbalance - you can have a high SWR with no imbalance; equally you can have a 1:1 SWR with high imbalance; they're different things!

Finally, for best results you really need to be using a ferrite core; iron powder will likely have high flux leakage.

If all that sounds too difficult, go back to the individual leg current sensors and wire them in series as per the VK5BR link.

Steve G3TXQ


Sun Jun 28, 2015 5:15 pm
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 Balancing an HF Doublet 
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Post Re: Balancing an HF Doublet
Now I am really confused.
So I have to put a 22 Ohm resistor across the winding on the toroid
then connect an ammeter after the detector to measure the current
without taking into account the current shunting effect of the 22 Ohm
resistor ??
For my RF ammeter I am using the same type of toroid that I use in
my balun.
The toroids are the big ones that are coloured red on one side and
black on the other side.
I'm not confusing balance and SWR.
Moving the current sensing toroid up and down the feeder will produce
different readings if the SWR is high.


Sun Jun 28, 2015 10:10 pm
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 Balancing an HF Doublet 
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Post Re: Balancing an HF Doublet
G8RFD wrote:
So I have to put a 22 Ohm resistor across the winding on the toroid
then connect an ammeter after the detector to measure the current
without taking into account the current shunting effect of the 22 Ohm
resistor ??

No - connect a DC voltmeter after the detector. The 22 Ohm resistor provides a defined load for the pick-up coil, which generates a voltage proportional to the sensed current:
Vdc = (Icm x 22 / N) - 0.5

G8RFD wrote:
Moving the current sensing toroid up and down the feeder will produce
different readings if the SWR is high.


If the line is balanced there will be no reading and no variation along the line whatever the SWR.
If it's unbalanced - yes the reading will vary along the line, but so will the individual leg currents. The ratio of the two - which is a measure of the degree of balance - should stay relatively constant along the line.

Steve G3TXQ


Sun Jun 28, 2015 11:08 pm
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 Balancing an HF Doublet 
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Post Re: Balancing an HF Doublet
OK thanks Steve.
I will do that and report back.
Sorry for my confusion.

I had a QSO on 40M today with DK5DR.
He said that my doublet will never work properly as it's a "Z" shape
and needs to be spread out in a straight line.
I guess he's probably right and I'm wasting my time and effort.
However I will bash on a bit longer so see what I can achieve.
Interesting to note that I was 5/9 in Germany so I guess my doublet
must be working reasonably well despite DK5DR's comments.
Food for thought.


Mon Jun 29, 2015 6:18 pm
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 Balancing an HF Doublet 
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Post Re: Balancing an HF Doublet
I wouldn't worry too much about those comments. Sure - a straight line is ideal, but sometimes you just have to do what you can. Bending it might drop the radiation resistance, but it wont stop it radiating.

But back to your start point - did you ever consider simply using a current balun rather than a voltage balun? If you use a voltage balun you have to adjust the antenna leg impedances to be identical if you want to balance the currents - that follows from Ohm's Law. But a current balun will drive balanced currents no matter what the antenna imbalance ...... within reason!

Steve G3TXQ


Mon Jun 29, 2015 7:12 pm
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 Balancing an HF Doublet 
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Post Re: Balancing an HF Doublet
Hi Steve,

Yes about 5 years ago I built a current balun.
This used windings of coax around ferrite rods.
I had strange results and found it simply didn't work.
I then built an air cored version and that was worse with RF in the shack.
After all my experiments, the only balun that worked properly was 11 turns
bifilar wound on the red and black micrometals toroid core as a voltage balun.
Any more or less than 11 turns wouldn't work as I couldn't achieve a good
SWR even with my Capco SPC 300 ATU.
I do realise that baluns don't like high SWR and I should really use an ATU with
a truly balanced output.
Thanks for your kind help and advice.


Mon Jun 29, 2015 8:07 pm
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 Balancing an HF Doublet 
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Post Re: Balancing an HF Doublet
Neither an air-cored nor a ferrite-rod-cored current balun is very effective on a multi-band antenna.

If ever you fancy "playing" some more, try a 1:1 current balun wound on ferrite toroids. 11 turns on two stacked FT240-52 toroids is very effective from 30m thru 15m; it's the 11th entry in this chart:
http://www.karinya.net/g3txq/chokes/

Steve G3TXQ


Mon Jun 29, 2015 8:16 pm
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