## Re: Foil based loop antennas

Expand Messages
• I ve used 1/4 self sticking copper foil used for stained glass work for quick magnetic loop antennas setup in hotel rooms. These run aorund \$5 for a 60ft
Message 1 of 16 , May 1, 2010
I've used 1/4" self sticking copper foil used for stained glass work for quick magnetic loop antennas setup in hotel rooms. These run aorund \$5 for a 60ft spool. I would run the foil along the hotel window spaced several inches inside the metal window frame. I used gamma matching at the bottom of the loop to match to the coax and a wide spaced air variable capacitor at the top where I split the foil. Using a battery operated soldering iron I tack soldered the wire connections to the foil. I also keep an assortment of capacitors in my bag in case I need to pad the tuning cap. Using up to 20 watts I worked 20-40M CW with good results. I've even worked as low as 80M if the window was large enough and also had surprisingly good results. When ready to check out of the hotel the foil comes off the window easily with no residue and I discard the foil in the trash.

Stealthy and effective!

Andrew
N3LCW

--- In loopantennas@yahoogroups.com, "sean" <bitterlemon40@...> wrote:
>
> I was thinking of making a loop antenna using copper foil. My understanding is that 100mm copper foil would be equivalent to 50mm copper pipe. However there is some talk about 'current crowding' at the edges of the foil.
• CQ magazine for April 1967 has an article Skin Effect in High Frequency Design . I quote: The values from the graph (fig. 1) represent the depth at which
Message 2 of 16 , May 1, 2010
CQ magazine for April 1967 has an article "Skin Effect in High
Frequency Design". I quote: "The values from the graph (fig. 1)
represent the depth at which currents will be reduced to 1/e (where
e=2.302) of the surface currents, and it is customary to provide for at
least 5 times the skin depth shown. For all practical purposes, 5 times is close to 100% conduction." The graph with the article is quite small,but it looks like the basic value for copper is 1 mil and for
aluminum, 2 mil. So if I interpret this correctly and the article is
true, 5 mils would suffice for copper, and 10 mil for aluminum. That
would seem to make copper foil usable. Aluminum foil, I suppose you
would have to use a much larger width to lower the overall AC resistance. I have thought about making a loop from plastics plumbing
pipe, and then somehow covering it with copper foil, like maybe a couple
lengths stretched along the whole length of the pipe, the loop being
built in a square. I wonder how solderable this copper foil is, if at
all. I found my two rolls of copper foil in a garbage dumpster after
some kind of constructon job ( we have a very wasteful country ) but I
think it is also sold for use in stained-glass hobby projects. Aluminum foil comes in widths up to maybe 18 inches and not expensive at all
so maybe it could be used somehow. You would have to build some
kind of box-like framework to attach it to, something looking kind of
like a box kite. -Hue Miller / Newport, Oregon, USA
• ... Hi Andrew A great idea. I m going to practice on the front picture window! By the way ... who caries the copper foil tape ? Thanks, Jim, VE3CI
Message 3 of 16 , May 1, 2010
At 12:37 AM 5/2/2010 +0000, you wrote:
>
>
>
>
>I've used 1/4" self sticking copper foil used for stained glass work for
>quick magnetic loop antennas setup in hotel rooms. These run aorund \$5 for
>a 60ft spool. I would run the foil along the hotel window spaced several
>inches inside the metal window frame. I used gamma matching at the bottom
>of the loop to match to the coax and a wide spaced air variable capacitor
>at the top where I split the foil. Using a battery operated soldering iron
>I tack soldered the wire connections to the foil. I also keep an
>assortment of capacitors in my bag in case I need to pad the tuning cap.
>Using up to 20 watts I worked 20-40M CW with good results. I've even
>worked as low as 80M if the window was large enough and also had
>surprisingly good results. When ready to check out of the hotel the foil
>comes off the window easily with no residue and I discard the foil in the
>trash.
>
>Stealthy and effective!
>
>Andrew
>N3LCW

Hi Andrew

A great idea. I'm going to practice on the front picture window! By the
way ... who caries the copper foil tape ?

Thanks,

Jim, VE3CI
• The foil I found in the trash is 2 inch width. Pretty nice stuff. Interesting the idea of the disposable loop antenna using narrower stuff. Now re current
Message 4 of 16 , May 1, 2010
The foil I found in the trash is 2 inch width. Pretty nice stuff.
Interesting the idea of the disposable loop antenna using narrower
stuff. Now re "current crowding", back around the late 1970s when
AEA (company ) was offering a loop antenna built on a metallic band
or stip, this may have been the reason MFJ company touted their loop
antenna as being more efficient than a model not using a cylindrical
conductor. Now that AEA is gone, I don't recall seeing MFJ advertisement
text that still claims this advantage, since there's not really any
competition. -Hue Miller
• It occurred to me that I could try a proof of concept experiment by taking a large cardboard box from a moving company, and running a strip of aluminum foil
Message 5 of 16 , May 1, 2010
It occurred to me that I could try a "proof of concept" experiment by
taking a large cardboard box from a moving company, and running a strip
of aluminum foil around it. That would be a strip around 12 inches or more wide. I wonder if she high voltage developed would be a fire hazard
with the cardboard? At least it would be safe at low powers. I recall
once many years ago, trying to run a 80 meters, 100 watt transmitter
into a short indoor antenna, made of aluminum foil by the way, and
setting the window curtain on fire, from the high voltage flashover.
-Hue Miller
• Your local home improvement store probably has copper tape as slug tape . The adhesive isn t conductive like EMI/EMC tape from 3M, but it s a lot more
Message 6 of 16 , May 1, 2010
Your local home improvement store probably has copper tape as "slug tape".
The adhesive isn't conductive like EMI/EMC tape from 3M, but it's a lot more
available.

73,

Brent - AB1LF

From: loopantennas@yahoogroups.com [mailto:loopantennas@yahoogroups.com] On
Behalf Of Jim Dunstan
Sent: Saturday, May 01, 2010 9:05 PM
To: loopantennas@yahoogroups.com
Subject: Re: [loopantennas] Re: Foil based loop antennas

At 12:37 AM 5/2/2010 +0000, you wrote:
>
>
>
>
>I've used 1/4" self sticking copper foil used for stained glass work for
>quick magnetic loop antennas setup in hotel rooms. These run aorund \$5 for
>a 60ft spool. I would run the foil along the hotel window spaced several
>inches inside the metal window frame. I used gamma matching at the bottom
>of the loop to match to the coax and a wide spaced air variable capacitor
>at the top where I split the foil. Using a battery operated soldering iron
>I tack soldered the wire connections to the foil. I also keep an
>assortment of capacitors in my bag in case I need to pad the tuning cap.
>Using up to 20 watts I worked 20-40M CW with good results. I've even
>worked as low as 80M if the window was large enough and also had
>surprisingly good results. When ready to check out of the hotel the foil
>comes off the window easily with no residue and I discard the foil in the
>trash.
>
>Stealthy and effective!
>
>Andrew
>N3LCW

Hi Andrew

A great idea. I'm going to practice on the front picture window! By the
way ... who caries the copper foil tape ?

Thanks,

Jim, VE3CI

[Non-text portions of this message have been removed]
• I bought several foil rolls on Ebay but they can also be found in arts and crafts stores that carry stained glass supplies. I found some locally after I
Message 7 of 16 , May 2, 2010
I bought several foil rolls on Ebay but they can also be found in arts and crafts stores that carry stained glass supplies. I found some locally after I bought mine on Ebay.

re; cardboard and fire: unless the loop arcs over somewhere or there are metal particles in the board I wouldn't worry about a few watts of RF. Personally, I'd avoid the cardboard anyway. It reminds me of the Harry Lythall SM0VPO packing crate loop antenna:

http://www.sm0vpo.com/antennas/crate.htm

Andrew
N3LCW

--- In loopantennas@yahoogroups.com, Jim Dunstan <jimdunstan@...> wrote:

>
> A great idea. I'm going to practice on the front picture window! By the
> way ... who caries the copper foil tape ?
>
> Thanks,
>
> Jim, VE3CI
>
• I would just like to clear up 2 common misconceptions: 1. Tubing vs foil (or any other non-close shape): RF current spreads out on the conductor and
Message 8 of 16 , May 2, 2010
I would just like to clear up 2 common misconceptions:

1. Tubing vs foil (or any other non-close shape):
RF current spreads out on the conductor and concentrates on
the edges. Tubing has no edge, therefore the RF
current will spread out more uniformly. If the tubing
is circular (as opposed to square) the current is forced by
symmetry to be uniform and there is no current crowding.
This is why round tubing will have the less loss than
foil of comparable dimensions.

2. Reducing conductor loss in a small loop ultimately results
in an impractically high Q with very narrow bandwidth. If
you are stuck with a small loop, it just isn't going to be
very efficient and there isn't a lot that you can do about it.

Summarizing: foil will be lossier than comparable tubing.
It may not be much better than wire. So your foil loop will
not be especially efficient. OTOH, if foil is convenient to
size than conductor size.

Rick N6RK
• I found on amazon.com JVCC brand 36 yard by 1/4, 1/2 and 1 1 oz (thickness measure) tape with conductive adhesive (USD \$12, 22, 39 respectively.)
Message 9 of 16 , May 2, 2010
I found on amazon.com JVCC brand 36 yard by 1/4, 1/2 and 1" 1 oz (thickness measure) tape with conductive adhesive (USD \$12, 22, 39 respectively.)

Maybe it is used in laptop display repair (for shielding the inverter and fluorescent lamp circuitry)?

Not known if it can be cleanly removed from window glass...

>>N3LCW
>
> Hi Andrew
>
> A great idea. I'm going to practice on the front picture window! By the
> way ... who caries the copper foil tape ?
>
> Thanks,
>
> Jim, VE3CI
>

_________________________________________________________________
The New Busy is not the old busy. Search, chat and e-mail from your inbox.
http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:WL:en-US:WM_HMP:042010_3
• Another one - 2 wide tape (thickness unspecified) also 36 yard roll, however, it does include in description text it s aggressive adhesive system... so
Message 10 of 16 , May 2, 2010
Another one - 2" wide tape (thickness unspecified) also 36 yard roll, however, it does include in description text "it's aggressive adhesive system..." so maybe not so easily removed? Anyway, USD 49 per 36 yard roll makes interesting possibility for wrapping lengths of PVC (not sure if it would survive outdoor flexing and getting around 45/90 degree angles...)

> I found on amazon.com JVCC brand 36 yard by 1/4, 1/2 and 1" 1 oz (thickness measure) tape with conductive adhesive (USD \$12, 22, 39 respectively.)
>
>
> Maybe it is used in laptop display repair (for shielding the inverter and fluorescent lamp circuitry)?
>
> Not known if it can be cleanly removed from window glass...
>
>>>N3LCW
>>
>> Hi Andrew
>>
>> A great idea. I'm going to practice on the front picture window! By the
>> way ... who caries the copper foil tape ?
>>
>> Thanks,
>>
>> Jim, VE3CI
>>
>
> _________________________________________________________________
> The New Busy is not the old busy. Search, chat and e-mail from your inbox.
> http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:WL:en-US:WM_HMP:042010_3

_________________________________________________________________
Hotmail has tools for the New Busy. Search, chat and e-mail from your inbox.
http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:WL:en-US:WM_HMP:042010_1
• Why go through the hassle? Wrapping copper tape on a PVC form leaves you with a fairly stiff loop, and alot of unknowns. Air con copper tube (available in
Message 11 of 16 , May 2, 2010
Why go through the hassle? Wrapping copper tape on a PVC form leaves you
with a fairly stiff loop, and alot of unknowns.

"Air con" copper tube (available in multiple sizes at any home supply or
plumbing house).

Thin wall copper pipe, easy enough to make an octagonal loop with a couple
sticks and some 45 elbows.

If you want flexable/portable there are a number of receive only designs
that used coax. You could also use a fine strand DC power cable, #4 can
be had that rolls up almost as easy as rope.

Losses would be higher, but I suspect losses would be lower in #4 power
cable than with copper foil wrapped on a PVC form.

Don't get me wrong, the copper foil trick is neat for a temp antenna, used
in myself on 6M once I just would not use it for a high current low band
antenna where I had another choice.

On Sun, 2 May 2010, david feldman wrote:

>
> Another one - 2" wide tape (thickness unspecified) also 36 yard roll,
> however, it does include in description text "it's aggressive adhesive
> system..." so maybe not so easily removed? Anyway, USD 49 per 36 yard
> roll makes interesting possibility for wrapping lengths of PVC (not sure
> if it would survive outdoor flexing and getting around 45/90 degree
> angles...)
>
>
>> I found on amazon.com JVCC brand 36 yard by 1/4, 1/2 and 1" 1 oz (thickness measure) tape with conductive adhesive (USD \$12, 22, 39 respectively.)
>>
>>
>> Maybe it is used in laptop display repair (for shielding the inverter and fluorescent lamp circuitry)?
>>
>> Not known if it can be cleanly removed from window glass...
>>
>>>> N3LCW
>>>
>>> Hi Andrew
>>>
>>> A great idea. I'm going to practice on the front picture window! By the
>>> way ... who caries the copper foil tape ?
>>>
>>> Thanks,
>>>
>>> Jim, VE3CI
>>>
>>
>> _________________________________________________________________
>> The New Busy is not the old busy. Search, chat and e-mail from your inbox.
>> http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:WL:en-US:WM_HMP:042010_3
• Hi All, If you tried to price the cost of copper lately, its clear that for an economic solution to the truely efficient magnetic loop antenna, a non-copper
Message 12 of 16 , May 16, 2010
Hi All,

If you tried to price the cost of copper lately, its clear that for an economic solution to the truely efficient magnetic loop antenna, a non-copper support of the loop makes sense. Really, to get good efficiency for middle HF frequency (7 to 15 MHz), a three inch diameter copper tubular conductor in the 3 to 4 foot diameter range is a good target.

A tubular conductor that size from copper, or even aluminum is very heavy, and will require some significant mechanical design.

A PVC octagon made from 3 inch or 4 inch lightwieght (not the solid) PVC tubing is a reasonable cost.

If one then, rather than wrap the copper flashing, but laid it side by side in-line with the PVC support tube , a low resitance loop could be made. The issue with wrapping , is that the currents flow in the direction of the tubing, and every time a current has to jump to the next layer of wrapping, it incurs a resistance. So, a wrapped configuration would require soldering every wrap-to-wrap joint, and then you still get the solder surface resistance. By having the conductors in line, with say a tack solder here and there, much less resistance is encurred.

Now, current bunching is an issue, but I have not seen any modeling to suggest how much of issue at HF. The practicalities of the flat loop may just not occur until much higher frequencies. Or maybe not. Without any true high frequency analysis, its just not clear how wide the strip has to be before the current bunching increases the resistance by what percentage over the predicted surface area resistance. Indeed, what modelling I have done on this issue tends to indicate the currents stay on the inside surface of a conductor with significant thickness, especially near the drive end of the loop. This implies that the optimum conductor may actually be a half-tube. Where current bunching pushes currents to the edges, but the induction action in the antenna pulls the currents torwards the smallest radius section of the tube. The result being an even coduction across the surface of the half-round conductor.
• ... The problem with this approach is that high efficiency necessarily results in very narrow bandwidth and very high Q (ie high voltages across the gap). At
Message 13 of 16 , May 16, 2010
bm40g wrote:
> If you tried to price the cost of copper lately, its clear that for an
> economic solution to the truely efficient magnetic loop antenna, a
> non-copper support of the loop makes sense. Really, to get good
> efficiency for middle HF frequency (7 to 15 MHz), a three inch diameter
> copper tubular conductor in the 3 to 4 foot diameter range is a good
> target.

The problem with this approach is that high efficiency necessarily
results in very narrow bandwidth and very high Q (ie high voltages
across the gap). At some point, it becomes impractical as an antenna.

> If one then, rather than wrap the copper flashing, but laid it side by
> side in-line with the PVC support tube , a low resitance loop could be

Right

> true high frequency analysis, its just not clear how wide the strip has
> to be before the current bunching increases the resistance by what

A tubular conductor uniquely has no edges for current to bunch up
on, and due to circular symmetry, the current is forced to be uniform.
Any non closed cross section will have current bunching.

> percentage over the predicted surface area resistance. Indeed, what
> modelling I have done on this issue tends to indicate the currents stay
> on the inside surface of a conductor with significant thickness,

What tool are you using to model? I did a bunch of measurements on
loops and found that the Q goes up as the conductor thickness increases,
but only to a point of diminishing returns. After that, you can't
get a higher Q no matter how thick the conductor. (This was for a
solid wire conductor). This maximum Q was only several hundred,
and could not be explained by radiation resistance. I hypothesized
that the currents were being bunched up by the magnetic field and
were not fully utilizing the conductor.

> especially near the drive end of the loop. This implies that the optimum
> conductor may actually be a half-tube. Where current bunching pushes
> currents to the edges, but the induction action in the antenna pulls the
> currents torwards the smallest radius section of the tube. The result
> being an even coduction across the surface of the half-round conductor.

That's interesting; two opposing effects. Hard to say which one "wins"

Rick N6RK
Your message has been successfully submitted and would be delivered to recipients shortly.