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Broadbanding a Link Antenna Coupler (lowering Q techniques?)




 
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Author Topic: Broadbanding a Link Antenna Coupler (lowering Q techniques?)  (Read 6877 times)
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aa5wg
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« on: January 06, 2010, 05:22:32 AM »

Hi to all:
I am home brewing a link antenna coupler and need some help.

What are some techniques electrically and or mechanically that would help increase broadbanding (lower effective opperating Q of link coupler) of the link antenna coupler?  I use this coupler in the series/current and parallel/voltage feed configuration for a random length doublet.

I read in an old ARRL handbook the series feed will take more inductance than parallel feed.  It did not mention how much more.  There was a note that the L/C ratio will be different for series vs. parallel.  Could someone explain this please with details and or examples.   

Bottom line, what are the techniques to lower operational Q of this link antenna coupler?

Thank you to all.

73,
Chuck Pool - AA5WG
Cedar, Michigan

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WA1GFZ
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« Reply #1 on: January 06, 2010, 10:33:15 AM »

Adding more L will decrease the Q. I have a pair of 22 uH inductors on my fugly tuner and the Q is pretty high on 160. 15 or 20 KHz qsy  and I need to retune.
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Steve - WB3HUZ
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« Reply #2 on: January 06, 2010, 10:34:35 AM »

I'm curious as to why you wish to lower the Q?

For a parallel-tuned system, when tuned to resonance, Q is the ratio of the load impedance and the capacitive reactance of the tuning capacitor.

For a series tuned system, when tuned to resonance, Q is the ratio of the capacitive reactance of the load impedance and the load impedance.

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aa5wg
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« Reply #3 on: January 06, 2010, 11:04:32 AM »

Hi WA1GFZ and Steve:
I would like to lower operating Q of Link Antenna Coupler to increase bandwidth usage before having to return the coupler.

I read in old handbooks that the coupler was built with a Q of 2.  Then some years later the books recommended a Q of 10.  I was wondering what I can do to controle Q?  I wanted to shoot for a Q of 4 or 5 but did not know how to calculate the inductance and capacitace values needed to do this.  This would be for series and parallel tuning.  Any ideas?

Thank you.
Chuck - AA5WG
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wa2dtw
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« Reply #4 on: January 06, 2010, 11:22:10 AM »

(suggestion deleted- I was thinking of an input circuit for a linear amplifier)
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KM1H
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« Reply #5 on: January 06, 2010, 11:25:05 AM »

Stainless steel antenna wire or a T2FD antenna.

Carl
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WA1GFZ
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« Reply #6 on: January 06, 2010, 11:49:59 AM »

True Carl, my antenna is #8 with #10 feeders. So if you use crappy feedline you can tune the world.
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KD6VXI
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« Reply #7 on: January 06, 2010, 01:39:16 PM »

Stainless steel antenna wire or a T2FD antenna.

Carl
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Steve - WB3HUZ
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« Reply #8 on: January 07, 2010, 12:08:42 AM »

Ok, less messing with the controls when QSYing makes sense. As you can see the Q is at least partially determined by the load at the input of the tuner. That load is of course determined by the antenna and frequency in use. So, there is no single setting or thing you can do that will work across bands. For any single band, appropriate selection of either series or parallel tuning will help make the Q lower.

The other thing you can do to change the Q (it may make it lower or not) is change the coupling between the primary and secondary of the link. A swinging or variable link or a capacitor in series with the primary can be used to change the coupling.

Or if you are using a parallel type tuner and are tapping down on the secondary with the feedline, always tap as far out towards the ends as possible to obtain a match. If you have to tap way down toward the center, consider changing the tuner arrangement to series feed.


Hi WA1GFZ and Steve:
I would like to lower operating Q of Link Antenna Coupler to increase bandwidth usage before having to return the coupler.

I read in old handbooks that the coupler was built with a Q of 2.  Then some years later the books recommended a Q of 10.  I was wondering what I can do to controle Q?  I wanted to shoot for a Q of 4 or 5 but did not know how to calculate the inductance and capacitace values needed to do this.  This would be for series and parallel tuning.  Any ideas?

Thank you.
Chuck - AA5WG
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KM1H
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« Reply #9 on: January 07, 2010, 11:54:00 AM »

Only until we get a hamband there Shane that allows unstable signals!

However the T2FD actually is a decent performing broadband antenna if the original Navy designs are followed. Over the years its been bastardized into the fairly lossy POS that is sold now buy the usual suspects.

Stainless steel is always lossy but for quick military and other deployments its proven quite popular and acceptable in a T2FD using a SS rig that now wont fold back power. 

Carl
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WA1GFZ
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« Reply #10 on: January 07, 2010, 12:05:28 PM »

Carl,
Where can I get some good information on that antenna. 
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k4kyv
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Don
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« Reply #11 on: January 07, 2010, 12:21:10 PM »

With parallel tuning (coil, capacitor and load all in parallel), increasing the capacitance and decreasing the inductance increases the Q.

With series tuning (load, coil, capacitor all in series), more inductance and less capacitance increases the Q.

This holds true with tuned links as well as the main tuned circuit in the antenna tuner.

I have always found it quicker and easier to use trial and error, than trying to calculate everything mathematically and then trying to build according to the calculations.

When I do calculations, I usually aim at the required capacitance and then use trial and error to find an inductance with appropriate physical dimensions that resonates with that calculated amount of capacitance.
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« Reply #12 on: January 07, 2010, 02:59:12 PM »

Sorta' like "The map is not the territory" axiom.... Wink
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aa5wg
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« Reply #13 on: January 07, 2010, 09:21:37 PM »

WA1GFZ, WB3HUZ, WA2DTW, KM1H, KD6VXI, K4KYV and all:
Thank you for your input.

Is there any other means to help lower operational Q of the link antenna coupler?
73,
Chuck Pool - AA5WG
Cedar, Michigan
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KM1H
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« Reply #14 on: January 07, 2010, 09:29:41 PM »

Frank

June 49 QST pg 54
Nov 51 CQ
Feb 53 CQ

Carl
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Steve - WB3HUZ
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« Reply #15 on: January 08, 2010, 12:14:57 AM »

Other than putting a resistor across it, no.   Grin


WA1GFZ, WB3HUZ, WA2DTW, KM1H, KD6VXI, K4KYV and all:
Thank you for your input.

Is there any other means to help lower operational Q of the link antenna coupler?
73,
Chuck Pool - AA5WG
Cedar, Michigan
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