At my summer QTH I mounted a simple wire delta loop, that I feed with a standard 300-home TV ribbon (I prefer symmetrical antennas, as having a good ground is not always that simple).

In the station I have an asymmetrical tuner, so, not to get RF on my radio, I must use a balun in between the tuner and the 300-ohm ribbon coming from the antenna.

I am not so enthusiast about using a common 1:1 or 4:1 toroidal transformer, as I am not sure on how those transformers behave across the very wide impedance range that they see at the ribbon end when operating 7 through 28 MHz.

So, I decided to instead use an RF choke that quenches the common mode RF. I firstly built a thick ferrite core by taping together five 3/8"OD ferrite rods, 6-inch long. I then made 10 turns of a very thick insulated red-black twin lead on it. The turns are widely spaced, so as to have low capacitance between adjacent turns.

The system works fine and gives me no problems, but I would like to hear some opinions on that approach, as I do not often see it proposed as a solution for feeding balanced lines.

Should I instead wish to try a common toroidal transformer, would you better use the 1:1 or the 4:1 version? My reasoning is as follows: unless my delta loop resonates on one of the ham bands, its impedance will generally be rather high. So, using a 4:1 balun would facilitate the tuner task. Any comment on that too?

Thanks and 73

Tony I0JX - Rome, Italy

On Thu, 27 Jul 2006 23:29:58 +0200, "Antonio Vernucci"
wrote:

So, I decided to instead use an RF choke that quenches the common mode RF. I firstly built a thick ferrite core by taping together five 3/8"OD ferrite rods, 6-inch long. I then made 10 turns of a very thick insulated red-black twin lead on it. The turns are widely spaced, so as to have low capacitance between adjacent turns.

The system works fine and gives me no problems, but I would like to hear some opinions on that approach, as I do not often see it proposed as a solution for feeding balanced lines.

Hi Tony,

Sounds good. The only unknown is the ferrite properties for the range
of RF.

Should I instead wish to try a common toroidal transformer, would you better use the 1:1 or the 4:1 version? My reasoning is as follows: unless my delta loop resonates on one of the ham bands, its impedance will generally be rather high. So, using a 4:1 balun would facilitate the tuner task. Any comment on that too?

You can also try putting a 1:1 current BalUn (or choke) on the coax
going from your rig to the tuner.

However, you do not say you had any problems to begin with. If you
are trying to achieve symmetry, you need to be able to measure how
close, or how far away you are. If you cannot do this, then you don't
know how much improvement you are making.

73's
Richard Clark, KB7QHC

"Antonio Vernucci" wrote
So, I decided to instead use an RF choke that quenches the common mode
RF. I firstly built a thick ferrite core by taping together five
3/8"OD ferrite rods, 6-inch long. I then made 10 turns of a very thick
insulated red-black twin lead on it. The turns are widely spaced, so
as to have low capacitance between adjacent turns.
=========================================

Congratulations. You have just re-invented the choke balun. It's what
many people use. It is the best choice. Very often it is wound on a
ferrite ring. It has no impedance or turns ratio.

Or it may consist of a number of ferrite sleeves which slide over the
transmission line.

It is a mistake to use a balun between tuner and transmission line
which has a definite impedance or turns ratio.
----
Reg.

Antonio Vernucci wrote:
My reasoning is as follows: unless my delta loop resonates on one
of the ham bands, its impedance will generally be rather high. So,
using a 4:1 balun would facilitate the tuner task. Any comment on
that too?

If the delta loop's impedance is rather high and the
feedline is an odd multiple of 1/4 wavelengths, what
impedance will the balun see?
--
73, Cecil http://www.qsl.net/w5dxp

"Reg Edwards" wrote in message
...

"Antonio Vernucci" wrote
So, I decided to instead use an RF choke that quenches the common
mode
RF. I firstly built a thick ferrite core by taping together five
3/8"OD ferrite rods, 6-inch long. I then made 10 turns of a very
thick
insulated red-black twin lead on it. The turns are widely spaced, so
as to have low capacitance between adjacent turns.
=========================================

Congratulations. You have just re-invented the choke balun. It's
what
many people use. It is the best choice. Very often it is wound on
a
ferrite ring. It has no impedance or turns ratio.

Or it may consist of a number of ferrite sleeves which slide over
the
transmission line.

It is a mistake to use a balun between tuner and transmission line
which has a definite impedance or turns ratio.
----
Reg.

======================================
A further comment -

It may not be obvious, but a number of ferrite sleeves sliding over
the transmissiom line, either coax line or twin line, corresponds to a
single turn of line passing through an elongated ferrite ring.

The formulae for calculating choking inductance, from dimensions and
number of turns, is the same for one ferrite ring and a single or a
number of sleeves.

Obviously, there is more heavy, ferrite material used for sleeves
because the number of turns is only one. But disregarding economics, a
number of sleeves will eventually provide a better very-wide-band
choke than a ferrite ring or a bundle of ferrite rods.

Rich, intelligent, technical perfectionists, using high power, with
plenty of spare time, are the most likely people to win contests.
----
Reg.

Reg Edwards wrote:
Obviously, there is more heavy, ferrite material used for sleeves
because the number of turns is only one. But disregarding economics, a
number of sleeves will eventually provide a better very-wide-band
choke than a ferrite ring or a bundle of ferrite rods.

Amidon specifies a typical Z for those toroidal coax sleeves.
For instance, the Z is 215 ohms per core for FB-77-5621
cores at 25 MHz. My store-bought G5RV came with eight of
those cores.
--
73, Cecil http://www.qsl.net/w5dxp