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Link Antenna Tuner - Faraday Shield




 
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Author Topic: Link Antenna Tuner - Faraday Shield  (Read 7615 times)
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aa5wg
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« on: August 08, 2012, 09:42:45 PM »

Hello to all:

I use a home brew link antenna coupler.  It is a tandem coil arrangement with one half the tank coil on each side of the link coil on a single form. 

Dose anyone have a good idea on how to incorporate/build an effective faraday shield between the split tank coil and link coil?

Chuck

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Steve - K4HX
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« Reply #1 on: August 08, 2012, 10:01:52 PM »

I don't have a good idea on the how to build the shield, but I would ask why you would want to build one?
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K5WLF
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« Reply #2 on: August 08, 2012, 10:40:19 PM »

I may be missing something (and probably am), but wouldn't that interrupt the coupling between the link and tank and make the whole thing not work?
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Steve - K4HX
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« Reply #3 on: August 08, 2012, 10:45:36 PM »

If it were a true shield it would. If you look at the ones used between links or other places, they always have gaps or slits cut in them.
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W7TFO
WTF-OVER in 7 land Dennis
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« Reply #4 on: August 08, 2012, 10:48:38 PM »

A 'finger' shield in a coupler will help keep radiated harmonics down.

73DG
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K5WLF
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« Reply #5 on: August 08, 2012, 10:51:46 PM »

A 'finger' shield in a coupler will help keep radiated harmonics down.

73DG

Thanks, Dennis. I'd not heard of the 'finger' shield. I was thinking the full "Faraday Cage" type shielding and just couldn't see how that was a good thing in a coupler.

ldb
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aa5wg
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« Reply #6 on: August 08, 2012, 11:04:10 PM »

One approach is to wind another coil over the link with a gap between the two.

On one side of this new winding solder a shorting bar across all the new turns and attach the ground connection to center.
.
On the opposite side of the shorting bar cut all the turns in half and allow a 1/4 inch gap.  

I wonder how effective two vertical faraday screens, one on each side of the link, would be?

Chuck
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Steve - K4HX
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« Reply #7 on: August 08, 2012, 11:41:37 PM »

Only if there is no stray coupling elsewhere in the circuit (include both the tuner and the feedline). At low freqs the stray coupling problem is likely minimal. 20-10 meters might be another story.

Given that a properly tuned link should have a bandpass characteristic anyway AND the tank in your transmitter SHOULD have a lowpass characteristic, is any additional harmonic reduction gained by a so-called Faraday screen really worth the effort?



A 'finger' shield in a coupler will help keep radiated harmonics down.

73DG
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W7TFO
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« Reply #8 on: August 09, 2012, 01:16:06 AM »

Given that a properly tuned link should have a bandpass characteristic anyway AND the tank in your transmitter SHOULD have a lowpass characteristic, is any additional harmonic reduction gained by a so-called Faraday screen really worth the effort?

Only if you can't keep a clean house the right way. Sad

It was popular in early TNT and other harmonic generators, I mean transmitters.

73DG
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The Slab Bacon
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« Reply #9 on: August 09, 2012, 08:48:00 AM »

IIRC, the Faraday Shield slould eliminate capacitive coupling between the windings, but not the magnetic coupling. So in theory, since the link-coupled tuna is actually a transformer, it should be totally un noticed at the tuned operating frequency.

A simple way to incorporate one into a link coupled tuna would be to wind the link out of small, stiff coass (like RG-142), use the center conductor for the link and ground the shield.

That is how it was done in a lot of older rigs. IIRC, either Budd or B&W offered a "kit" to do it that way for their larger plug-in coils.
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W3GMS
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« Reply #10 on: August 09, 2012, 09:24:08 AM »

B&W offered a Faraday shielded link to go with their big HDVL plug in coils.  The shield was such that you could take it apart to vary the turns on the link.  Of coarse the shielded link had an insulator in its construction so it did not look like a shorted turn.  It was introduced around the time TVI was rearing its ugly head! 

Joe, W3GMS     
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WA1GFZ
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« Reply #11 on: August 09, 2012, 09:49:29 AM »

Frank, that type of shielded primary link worked because the shield was only terminated to ground at one end. the other end of the link it floated. Not really necessary as Steve indicated. I think you would be better off running the coax into the tuner through a string of beads to reduce common mode currents on the coax.
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aa5wg
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« Reply #12 on: August 09, 2012, 11:22:41 AM »

WA1GFZ:

What value or type, size and how many beads are recommended to place on coax at input of tuner?

Chuck
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The Slab Bacon
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« Reply #13 on: August 09, 2012, 11:53:49 AM »

Frank, that type of shielded primary link worked because the shield was only terminated to ground at one end. the other end of the link it floated. Not really necessary as Steve indicated. I think you would be better off running the coax into the tuner through a string of beads to reduce common mode currents on the coax.

Frank,
         I thik the purpose of the faraday shield in a link-coupled tuna or final tank was to reduce / eliminate any capacitive coupling that would let harmonics or VHF spurious emissions "leak" out to the antenner and cause TVI. I don't know if magic beads would have the same effect. (although it probably wouldn't hurt).

But....................... In this day of now digital everything and cable / fiber optic TV, I really don't see it being necessary anymore.
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WA1GFZ
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« Reply #14 on: August 09, 2012, 12:13:17 PM »

you need at least 200 ohms of reactance on lowest band. 15 to 20uh should do it

The TVI radiation would come off the coax shiels so a common mode choke will work.
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KF1Z
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Are FETs supposed to glow like that?


« Reply #15 on: August 09, 2012, 01:11:17 PM »

Isn't there some benefit of reducing noise from things like static discharge etc ?

I've heard some claim to reduce RX noise considerably in this fashion.

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aa5wg
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« Reply #16 on: August 09, 2012, 01:48:43 PM »

Guys:

If the transmitter is clean I don't see the need for any beads at the input of the link antenna tuner.

What do you think?

Chuck
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WA1GFZ
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« Reply #17 on: August 09, 2012, 02:13:51 PM »

Bruce has a point if there is any common mode crud on the shield. Also beads will limit shield current if there is any reflection from the link
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KM1H
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« Reply #18 on: August 09, 2012, 04:57:16 PM »

Quote
you need at least 200 ohms of reactance on lowest band. 15 to 20uh should do it

The TVI radiation would come off the coax shiels so a common mode choke will work.

You need at least 10X Zo to be effective and preferably more. The easiest (and cheapest) way is to get a pair of 31Mix 2.4" toroids from Mouser and wrap it full of coax which will get you a couple of thousand Ohms XL at 160 and peaking around 5K by 10M. Wind it loose with the coax taking wide loops.
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aa5wg
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« Reply #19 on: August 09, 2012, 10:10:58 PM »

What do you recommend for high quality slip on type of choke that goes over the coax, i.e. type, make, size, power?

Chuck
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WA1GFZ
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« Reply #20 on: August 10, 2012, 11:04:10 AM »

Yes 10X is better I'm thinking of BB transformers. Snap on just buy one and measure the inductance of 1 turn. Calculate the reactance then determine how many you need.
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Steve - K4HX
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« Reply #21 on: August 12, 2012, 09:19:11 PM »

As Frank noted, there may be some common mode noise reduction (locally generated stuff) but there will be no reduction from lightning static.



Isn't there some benefit of reducing noise from things like static discharge etc ?

I've heard some claim to reduce RX noise considerably in this fashion.


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w1vtp
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« Reply #22 on: August 12, 2012, 10:56:10 PM »

I may be stating the obvious but the purpose of the Faraday shield is to reduce capacitive coupling in a link coupling so that harmonics that might normally be passed on from the tank to the link coil might be reduced.  This harmonic capacitive transfer might be a problem with class "C" amplifiers.  Modern transmitters have to pass rigid type acceptance specifications.  The latter being the case, I wonder if a Faraday shield would be necessary.

HB transmitters especially class "C" amps might need such measures to reduce harmonic content.  That said, expert homebrew builders like to use the National MB 40 that includes a Faraday shielded link [see attachment] in grid circuits that reduces harmonic content to their final amplifiers just to keep ahead of such a problem.  One other method of Faraday shielding is a single loop of coax used as a link where the center conductor of the coax used in this link is terminated to the shield at the end of the single turn with no connection of the end of the shield to anything - there are examples of this technique in various amateur radio handbooks.

Modern transmitters rely on the PI network to reduce harmonic output.  My old FT-301s QRP transceiver uses double Pi filtering in both the output of the transceiver and the FL-110 100 watt amplifier.  In that case there would probably not be a need for the Faraday shield in a tuner.  However, should I hook this up to a class "C" amplifier I might then need such a shield to reduce harmonic content that the amplifier generates.

Finally, the handbook recommends fabricating a Faraday shield by attaching a series of parallel wires to an insulative board and which is terminated at one end to a buss wire which, in turn, is grounded. The shield needs to be larger than the coils and of course each side of the link coil needs to have this shield.

Al

PS: Filesize was reduced to help download time.


* MB40 SL WITH FARADAY SHIELD comp.jpg (388.29 KB, 2328x1577 - viewed 438 times.)
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