Coiled coax balun
 

Hi again!

Has anyone tried making the coiled coax balun work at UHF? Specifically, on
the 70 cm band.

In "Antennas for All Applications", they say the coil should be resonant at
the frequency of operation. The ARRL book(s) do not cover this band. Using
RG58 and a GDO, I've learned that one cannot leave long leads on one's coax
coil as it will lower the resonant frequency drastically. Apparently, the
long leads in conjunction with the coil forms an antenna/inductor/capacitor
combination.

It appears to be difficult to obtain the required resonance repeatably. Or,
am I missing something (as usual)?

The diameter seems to be somewhat critical. Does the diameter need to be
much, much smaller than a quarter wave? What diamter would be a maximum?

Thanks for any help.

John (KD5YI)

On Sun, 26 Sep 2004 15:24:40 -0500, "John Smith"
wrote:

Has anyone tried making the coiled coax balun work at UHF? Specifically, on
the 70 cm band.

Hi John,

Not when ferrites or resonant sections do the job too.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Sun, 26 Sep 2004 15:24:40 -0500, "John Smith"
wrote:

Has anyone tried making the coiled coax balun work at UHF? Specifically,
on
the 70 cm band.

Hi John,

Not when ferrites or resonant sections do the job too.

73's
Richard Clark, KB7QHC

Hi, Richard -

Please give me more information about the ferrites...

How many do I need?
What permeability?
Who makes them capable of 450 MHz or so?

Thanks.

John

"John Smith" wrote in message
...
Hi again!

Has anyone tried making the coiled coax balun work at UHF? Specifically,
on
the 70 cm band.

In "Antennas for All Applications", they say the coil should be resonant
at
the frequency of operation. The ARRL book(s) do not cover this band. Using
RG58 and a GDO, I've learned that one cannot leave long leads on one's
coax
coil as it will lower the resonant frequency drastically. Apparently, the
long leads in conjunction with the coil forms an
antenna/inductor/capacitor
combination.

It appears to be difficult to obtain the required resonance repeatably.
Or,
am I missing something (as usual)?

The diameter seems to be somewhat critical. Does the diameter need to be
much, much smaller than a quarter wave? What diamter would be a maximum?


By the time you get to UHF it is difficult to use coils. Baluns are usually
made of coax or some of the very small hard line like less than 1/4 inch in
diameter. The coax is just a fraction of a wavelength.

On Sun, 26 Sep 2004 19:38:02 -0500, "John Smith"
wrote:
Please give me more information about the ferrites...

Hi John,

Visit:
http://bytemark.com/products/content2.htm
You will find background information here, but unfortunately they
don't maintain the pages very well and some information is lacking.

How many do I need?

Depends on application. You want at least 3 times the expected load
Z. Ferrite Z is bulk determined and the usual standard of bulk size
is defined as a type 101 size bead.

What permeability?

Type 43 or 64 material. 64 being slightly better (by about 15%).

Who makes them capable of 450 MHz or so?

Amidon, but this is not proprietary (although the type specification
may be). There are other manufacturers like Fairite.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
. . .

Who makes them capable of 450 MHz or so?


Amidon, but this is not proprietary (although the type specification
may be). There are other manufacturers like Fairite.


I don't believe that Amidon manufactures cores (except, apparently,
custom ones through one if its subsidiaries), but only resells them. The
last I checked, most of their ferrite cores are from Fair-Rite. The
common numerical ferrite type designations of 43, 72, and so forth are
Fair-Rite's.

They also sell powdered iron cores from Micrometals.

Roy Lewallen

"Ralph Mowery" wrote in message
nk.net...

"John Smith" wrote in message
...

By the time you get to UHF it is difficult to use coils. Baluns are
usually
made of coax or some of the very small hard line like less than 1/4 inch
in
diameter. The coax is just a fraction of a wavelength.

Hi, Ralph -

Yes, I can see that now. A little over two turns of RG58, tightly wound, on
a .75 inch diameter is about right to be resonant around 440 MHz. However,
the resonant frequency moves with compression and expansion of the coil (not
unexpectedly) making it difficult to settle on the dimensions.

The problem here is that I must have no leads on the coil while
experimenting because the leads affect the frequency. Later, when I create
the coil at the base of the antenna, I will not be able to determine the
coil's exact resonant frequency and adjust it for my frequency of operation.

I think I'll give up on this idea.

Thanks for your comments.

John

"Richard Clark" wrote in message
...
On Sun, 26 Sep 2004 19:38:02 -0500, "John Smith"
wrote:
Please give me more information about the ferrites...

Hi John,

Visit:
http://bytemark.com/products/content2.htm
You will find background information here, but unfortunately they
don't maintain the pages very well and some information is lacking.

How many do I need?

Depends on application. You want at least 3 times the expected load
Z. Ferrite Z is bulk determined and the usual standard of bulk size
is defined as a type 101 size bead.


Thanks for the info, Richard.

Is there a way to add toroids to RG58 and measure the resulting
effectiveness? Maybe measure the RF on the shield with an RF Voltmeter?

John

On Mon, 27 Sep 2004 09:36:21 -0500, "John Smith"
wrote:

Is there a way to add toroids to RG58 and measure the resulting
effectiveness? Maybe measure the RF on the shield with an RF Voltmeter?

Hi John,

Apply a steady, known power to a resistor (25 - 75 Ohms) and measure
the voltage (more for sanity's sake). Add one core to the lead (make
leads as short as possible, but long enough to allow ONE core to be
added. Measure the drop across BOTH the resistor AND the core.
Measure the drop across EACH, the resistor and the core. Across the
core means at the wire passing through the center, as if the core were
a leaded component with leads emerging from either side (it will NOT
act like a short! - ain't science wunnerful?). Standard ratiometrics
will reveal the core's Z.

You need only measure one core at a time, the addition of cores is
additive just like series resistors. Unlike chained resistors,
successive passes of the same wire through the core raises the Z by
the square of the turns. A "turn" is defined as each passage of a
conductor through the center (do not confuse with loops which may give
rise to the appearance of a "turn" that in fact does not have a lead
through the center; or that you lose a turn by counting only loops).

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Mon, 27 Sep 2004 09:36:21 -0500, "John Smith"
wrote:

Is there a way to add toroids to RG58 and measure the resulting
effectiveness? Maybe measure the RF on the shield with an RF Voltmeter?

Hi John,

Apply a steady, known power to a resistor (25 - 75 Ohms) and measure
the voltage (more for sanity's sake). Add one core to the lead (make
leads as short as possible, but long enough to allow ONE core to be
added. Measure the drop across BOTH the resistor AND the core.
Measure the drop across EACH, the resistor and the core. Across the
core means at the wire passing through the center, as if the core were
a leaded component with leads emerging from either side (it will NOT
act like a short! - ain't science wunnerful?). Standard ratiometrics
will reveal the core's Z.

You need only measure one core at a time, the addition of cores is
additive just like series resistors. Unlike chained resistors,
successive passes of the same wire through the core raises the Z by
the square of the turns. A "turn" is defined as each passage of a
conductor through the center (do not confuse with loops which may give
rise to the appearance of a "turn" that in fact does not have a lead
through the center; or that you lose a turn by counting only loops).

73's
Richard Clark, KB7QHC

Thanks, Richard. I'll give this a shot. Might not get to it until the
weekend, though.

73,
John