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Author Topic: RF shunt coil for mobile screwdriver antenna  (Read 4858 times)
N5UD
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Posts: 800




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« on: March 09, 2012, 07:12:03 PM »

As simple as this sounds I have come across a problem with mine. The coil that I wound is #10 copper, 1 1/4 diameter, 10 turns. Yes I stretched it, messed with a tap, used antenna analyzer, and so on. I got the compromise match under 1.5 :1 on 40 and 80. Now I just discovered this coil has loss and de-tunes the screwdriver antenna on 10 meters. Some small amount of reaction on 12 meters. Other bands OK.

OK how do I know this ? I made tests with shunt coil mounted versus removed. I checked both received signal and transmitted signal. Receive is about 1 S unit. I can tune the antenna on 10 meters and check with analyzer on either configuration OK.

I have not wound another coil. I just now thought I would throw this out here to the group for a known good coil. Must be 500 watt rating. I used 10 as that's what I had laying around.

Heck maybe I am missing something ?

73 Tony N5UD
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G8YMW
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Posts: 219




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« Reply #1 on: March 10, 2012, 02:58:36 AM »

Perhaps it is that 28 MHz is a short enough wavelength not to need it?? Just thinking about CB, they dont need it.
Thinking aloud
A physically short aerial at resonance has a lower impedance so % shorter= lower impedance.
My understanding is that a shunt match is to get the impedance back up towards 50 Ohm.

I will leave it to more articulate people to explain better.

Cheers Tony
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73 details Tony
Sent by WW2 Royal Navy signal lamp
G8YMW
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Posts: 219




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« Reply #2 on: March 10, 2012, 03:09:07 AM »

Perhaps the ultimate answer is to have a separate aerial for ten (maybe twelve) ?
Alot of faffing about.
Switching the shunt coil in and out?

"Nurse!! My brain hurts!! ( With apologies to Monty Python)

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73 details Tony
Sent by WW2 Royal Navy signal lamp
WV4L
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Posts: 137


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« Reply #3 on: March 10, 2012, 05:20:34 AM »

I remember reading some where if you do as you did with this shunt you loose 10M and possibly 12M in the process. Perhaps using 2 different shunts employing a relay to switch between the two is possible.
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WX7G
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Posts: 5977




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« Reply #4 on: March 10, 2012, 06:33:54 AM »

With your coil close-wound it works out to be 2.4 uH at 3.75 MHz. On 28 MHz it is below self-resonance and presents an impedance of 475 ohms inductive. This should not significantly affect the 28 MHz match but apparently it does. I ran a coil program and a different coil length-to-diameter ratio raises the self-resonant frequency.

I see four solutions:

1) Wind a coil having a diameter of 0.75 inches. It will need about 20 turns.

2) Disconnect one end of the shunt coil when operating on 10 meters.

3) Mount an MFJ-910 Mobile Antenna Matcher at the antenna and manually switch the shunt capacitance as needed. MFJ also makes a shunt inductance box, the MFJ-908.

4) Use an MJF-907 matching transformer. For 80/40 meters it should work ok mounted at the radio. On the higher bands set it to 1:1.



« Last Edit: March 10, 2012, 09:13:53 AM by WX7G » Logged
W5DXP
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« Reply #5 on: March 10, 2012, 06:38:10 AM »

Heck maybe I am missing something ?

Are you using a coil form? Insulated wire? Tape or shrink-wrap over the coil? How close to the chassis is the coil? Your coil may be experiencing transmission line effects. In addition to self-capacitance, capacitance to the surroundings can cause a resonance effect rather than an inductive reactance effect. The solution is to disconnect the coil for operation above 21 MHz.

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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
N5UD
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Posts: 800




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« Reply #6 on: March 10, 2012, 08:49:54 AM »

The solution....disconnect the coil on 12 and 10. This I have been doing. My ground point for one end of the coil is via a screw. So I keep the hex driver handy.

Capacitance near the coil......coil is about 1 foot from any body parts. The coil is about one inch below the base of the antenna.

I recall reading more than once that shunt coils are "invisible to the higher frequencies". You can't prove it by me.

The matter is still open for discussion.

Thanks  73 N5UD
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N5UD
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Posts: 800




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« Reply #7 on: March 10, 2012, 08:54:33 AM »

Ohhhh..... failed to comment on this. The coil is open air bare copper 10 ga wire. No nothing added.
N5UD
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WX7G
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« Reply #8 on: March 10, 2012, 09:14:53 AM »

See my updated post on a different coil length-to-diameter ratio to try.
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N5UD
Member

Posts: 800




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« Reply #9 on: March 10, 2012, 08:15:09 PM »

See my updated post on a different coil length-to-diameter ratio to try.

1) Wind a coil having a diameter of 0.75 inches. It will need about 20 turns.

Sorry I saw the MFJ stuff and missed this. What gauge wire ? 12 or so ?

I will add that I need the shunt coil in order to match 30 meters closer than without it. Absolutely have to use the coil for 40 and 80. The antenna tunes 20 through 10 FB without the shunt coil.

Thanks N5UD
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WX7G
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Posts: 5977




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« Reply #10 on: March 11, 2012, 04:59:37 AM »

Any size wire will be fine. The reason for the different coil length-to-diameter ratio is to raise the self-resonant frequency of the coil.

But according to the coil program the original coil is not self-resonant at or below 28 MHz so what else could the problem be? As W5DXP says it might be too close to other objects. Can you place the coil a couple of inches away from things?

I tried to use a shunt coil for the Tarheel screwdriver I have in the backyard but it was so much trouble I switched to the DX Engineering matching transformer.
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N5UD
Member

Posts: 800




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« Reply #11 on: March 11, 2012, 06:45:35 PM »

I may just get a clip, like on bug catcher taps. When I need the coil, 30M - 80M, I will attach coil to the ground.

Rained all day so no chance today to play with it.

N5UD
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