Hi all.

Throwing together a 20m single quad loop (triangular) antenna. In the
past I have fed these with a 75r Q section. Nothing strange here.. I use
to coil up most of it in a 5" coil to help reduce the line radiation
problem - if any... No tuner in use..

This time however I am going to use two pieces of 50 ohm coax in
parallel giving a 100r balanced Q section feeder. This more closely
matches the loop Z. The shields are joined together at each end of the
section and the coax centre conductors become the two antenna connection
points.

Question;

Should I be worried about inbalance in the 50 ohm cable from the radio
to the Q section. ie does the Q section reduce any of that problem? If
that isnt the case I can make up the usual coil of coax before it meets
the Q section.. or can I coil up the Q section instead.

I also wonder about Q section shield grounding as I hadnt intended
grounding either end. Will this be an issue?

Cheers Bob W5/VK2YQA

On Mon, 12 Sep 2005 15:39:23 -0500, Bob Bob wrote:

Hi all.

Throwing together a 20m single quad loop (triangular) antenna. In the
past I have fed these with a 75r Q section. Nothing strange here.. I use
to coil up most of it in a 5" coil to help reduce the line radiation
problem - if any... No tuner in use..

This time however I am going to use two pieces of 50 ohm coax in
parallel giving a 100r balanced Q section feeder. This more closely
matches the loop Z. The shields are joined together at each end of the
section and the coax centre conductors become the two antenna connection
points.

I have a question, Why?

The loop Z is going to be in the neighborhood of 120 ohm. A 75 ohm Q
section is a better deal than 100 ohm.



Question;

Should I be worried about inbalance in the 50 ohm cable from the radio
to the Q section. ie does the Q section reduce any of that problem? If
that isnt the case I can make up the usual coil of coax before it meets
the Q section.. or can I coil up the Q section instead.

I also wonder about Q section shield grounding as I hadnt intended
grounding either end. Will this be an issue?

Cheers Bob W5/VK2YQA

The modelled Z is around 140 ohms. The "top" element of the loop is
about 20% longer than the other two. It is starting to approach a folded
dipole.

Bob

Wes Stewart wrote:
I have a question, Why?

The loop Z is going to be in the neighborhood of 120 ohm. A 75 ohm Q
section is a better deal than 100 ohm.

On Mon, 12 Sep 2005 18:38:34 -0500, Bob Bob wrote:

The modelled Z is around 140 ohms. The "top" element of the loop is
about 20% longer than the other two. It is starting to approach a folded
dipole.

Okay. 75 ohm is -still- better than 100.

Err why?

50r to 100r = 122r
50r to 75r = 90r

Do I have the calcs right?

Wes Stewart wrote:

Okay. 75 ohm is -still- better than 100.

Bob Bob wrote:

Err why?
50r to 100r = 122r

SQRT(50*200) = 100

50r to 75r = 90r

SQRT(50*100) = 70.7

Do I have the calcs right?

Wes Stewart wrote:
Okay. 75 ohm is -still- better than 100.

SQRT(50*140) = 84 ohms. Z0=75 ohms is closer to
84 ohms than is Z0=100 ohms. If you had a 200 ohm
feedpoint, Z0=100 ohms would be perfect.
--
73, Cecil http://www.qsl.net/w5dxp

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On Tue, 13 Sep 2005 10:04:56 -0500, Bob Bob wrote:

Err why?

50r to 100r = 122r
50r to 75r = 90r

Cecil already gave you the answer. A 90-degree line is called a
"geometric mean" transformer.

The required impedance is the square root of the product of the
desired R and the load R.

Here's a little tutorial:

http://www.microwaves101.com/encyclo...uarterwave.cfm