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Author Topic: Magloop: low-RF-resistance mechanical connections?  (Read 3049 times)
JAHAM2BE
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« on: May 20, 2012, 01:58:34 AM »

Loss resistance in a magnetic loop antenna must be kept to a minimum due to the low radiation resistance of the antenna. This suggests all-soldered or all-welded construction.

On the other hand, there are times when one would like to disassemble a small transmitting loop antenna for easier transport into the field.

For example, the AlexLoop Walkham antenna uses what appear to be gold-plated N-connectors to connect a coaxial cable loop element to the capacitor.
Image: http://www.alexloop.com/images/walkham/img011.jpg
Description: http://www.alexloop.com/instructions2.html

As another example, the MFJ loop tuners just seem to use wingnuts:
Image: http://frrl.files.wordpress.com/2009/06/mfj-936b_image_back.jpg
Description: http://frrl.wordpress.com/2009/06/07/small-transmitting-loop-tuners-from-mfj/

Naturally, any mechanical connection in a small loop will introduce loss, but the advantage in portability may be worth it in some cases. I'm thinking about using some hardline coaxial cable for the loop element plus some mechanical, low-loss way of connecting the coax to the capacitor. That might make a good compromise antenna (somewhat lossy, but not overly) that could be taken into the field for portable operations from a good location.

Has anyone had any experience with - or perhaps even measured the RF resistance of - mechanical connections that work well in a small transmitting loop antenna? Note this will be for QRP use (5 watts from an FT-817).
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WB4SPT
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« Reply #1 on: May 20, 2012, 07:03:56 AM »

I've done a bit of study on metal resistance over the years.  Supreme rule is to keep stainless steel out of the conductive path.  Use pure copper whenever possible, UNLESS you are fastening to Al.  If you really need more strength than copper, then go to silicon bronze hardware, it has about 90 +/- % of Cu, but is some 2 to 3x the resistance of pure copper.  There is also a paste to apply to keep O2 out of the equation, I think GA Cu has it. 
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AB5Q
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« Reply #2 on: May 20, 2012, 07:07:26 AM »

The AlexLoop antenna uses Au (gold) plated PL-259 connectors, the key factor in minimizing R loss is the Au plating. Au does not easily oxide, has a low bulk resistance and will therefore remain at a very low R over the service life of the connector. The DC resistance of a good quality PL259, N, BNC or even SMA connector with Au contacts is on the order of tens of milliohms, so I wouldn't be too concerned about the R of the connection when using Au contacts. I would be concerned with the mechanical strength of the connector to properly support the weight of the loop without distorting the connector. Although mechanically robust, a N type connector is more difficult to mate than PL-259, so that needs to be taken into consideration.  In my opinion the best possible scenario electrically and mechanically would be 1/2" Heliax with N connectors. However, the added expense using this combination may not be worth the performance advantage.

In summary, to minimize R loss or contact resistance, select a connector with Au plated contacts. 

73 - John
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WB6BYU
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« Reply #3 on: May 20, 2012, 08:32:46 AM »

The original 1960's "Army Loop" used in Vietnam had 8 pieces of aluminum tubing in an
octagon shape. These connected together using 45 degree "elbows" (135 degrees, actually).

They found they had to gold-plate the joints to get reliable performance, otherwise
any bit of corrosion would cause the joint resistance to increase.

So I'd look for some gold-plated PL-259 connectors.  Then if you are using hardline
strip back the insulation and solder the shield directly to the outside of the connector
shell (assuming that the shield is too big to fit inside the connector in the normal
fashion.


For ham use, you might get adequate performance with silver plated joints - there are
several ways this can be done at home.  In that case you could use copper plumbing
tubing and connectors (usually made by flaring out the end of the tubing and clamping
it in place with a nut.)  You can silver plate just the ends of the tubing where they
connect.  Of course, you'll have to insulate the connectors from the mounting box
somehow.
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W0BTU
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« Reply #4 on: May 20, 2012, 02:58:05 PM »

The Alexloop's maximum power rating is only 20W PEP, 10W continuous. That tells me that it probably gets pretty warm somewhere when used for transmitting. What's the weak point, that is, what part fails if we go over that limit?

As for gold, the advantage is not that it is a better conductor than silver. Gold is just more resistant to corrosion and therefore is more reliable.

Silver is actually a better conductor than gold; and if we take the time to pay attention to the condition of the contacts (that is, keep it free from tarnish), it may be the material of choice.

http://www.eddy-current.com/condres.htm shows the relative conductivities of metals. Clean silver has roughly 35% better electrical conductivity than gold. Suit yourself.
« Last Edit: May 20, 2012, 03:04:46 PM by W0BTU » Logged

VE7TIT
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« Reply #5 on: May 20, 2012, 03:38:05 PM »

> The Alexloop's maximum power rating is only 20W PEP, 10W continuous. That tells me that it probably gets pretty warm somewhere when used for transmitting. What's the weak point, that is, what part fails if we go over that limit?

The limiting component is probably the voltage rating on the capacitor. You get some mighty high voltages with these things.


> Silver is actually a better conductor than gold; and if we take the time to pay attention to the condition of the contacts (that is, keep it free from tarnish), it may be the material of choice.

If memory serves, silver tarnish is silver sulfide, which has a higher conductivity than silver. Do I have that right? If so, polishing silver connectors would take things the wrong way.

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W0BTU
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« Reply #6 on: May 20, 2012, 04:09:48 PM »

You're probably right about the capacitor. I should have thought of that, since I published a web page about this with a capacitor voltage chart on it.

Silver normally forms an oxide tarnish. Silver sulfide happens in the presence of hydrogen sulfide gas, which is present in a lot of well water. Last place I lived in Ohio, there was only the faintest hint of sulfur in the water, but it sure degraded my unprotected indoor silver-plated contacts and connectors.

I think clean, bright silver is the best conductor.
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JAHAM2BE
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« Reply #7 on: May 23, 2012, 08:41:51 PM »

I'm thinking of building a loop antenna similar to VK2ZAY's bicycle rim antenna:

Image: http://www.vk2zay.net/article/file/677
Close-up image: http://www.vk2zay.net/article/file/678
Schematic: http://www.vk2zay.net/article/file/680
Article: http://www.vk2zay.net/article/158

The design is a capacitive match (Army loop) using binding posts to connect the tuning capacitor to the loop, and to connect fixed resonating capacitors (silver mica) across the variable resonating capacitor. I have a small butterfly cap (25pf stator-to-stator) that I would use instead of VK2ZAY's dual-gang/split stator cap. Also my loop conductor will be copper (tubing, strap, or wire).

Given this design, do any of you have suggestions for how to minimize losses? From the above discussion, I'm thinking I could get silver- or gold-plated binding posts, flatten (to maximize surface area) and silver-plate the loop conductor where it meets the binding posts, and of course make the loop area as large as possible to maximize radiation resistance.

Anything else? Some kind of conductive paste, maybe? Can conductive paste be used on a connection that is repeatedly loosened and tightened? VK2ZAY also mentions the use of star washers. Would those be advisable?

I'm also worried about losses in the fixed silver mica caps and the tiny connecting leads to the fixed caps (which are also mechanically connected via the binding posts), but they are cheap and mechanically easy to mount. Probably a larger fixed capacitor with larger connectors would be better.
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JAHAM2BE
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« Reply #8 on: June 01, 2012, 11:39:09 AM »

How about this idea for a homebrew low-loss connector?

http://i46.tinypic.com/epmxqb.png

We have two pieces of copper tubing of equal diameter. The diameter of the left-hand tube is slightly widened with a swaging tool like this: http://www.sweethaven02.com/BldgConst/en5111aa0027.gif (from http://www.free-ed.net/sweethaven/bldgconst/plumbing01/lessonmain.asp?iNum=fra0203 in section "Preparing for swaged joint"). A slit is cut in the enlarged left-hand tube, and it is slid over the right-hand tube for a snug fit. A hose clamp is placed around the joint and tightened.

In the particular application of a magnetic loop antenna, one piece of tubing would be soldered to (or part of) the loop radiator, and one piece would be soldered to the loop capacitor.

Does this look like it could be a low-loss connection at HF frequencies with high currents as found in a magnetic loop antenna?
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W0BTU
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« Reply #9 on: June 01, 2012, 01:02:00 PM »

How about this idea for a homebrew low-loss connector? ... Does this look like it could be a low-loss connection at HF frequencies with high currents as found in a magnetic loop antenna?

Certainly.

Woven braid is a terrible thing to use in an STL, because of the losses. Copper straps are FAR better, and the swaged copper item you describe should be even better than a strap.
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WB6BYU
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« Reply #10 on: June 01, 2012, 01:30:49 PM »

Even better if the contacts are silver plated.
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