Narrow Foil Strips for Ground Plane

In article ,
Dennis Mountains wrote:
Hi,

I'm building a Lancair Legacy (all carbon fiber) and am planning to put
copper foil strips on the belly to serve as a ground plane for the
belly-mounted com antenna. I'm going to use 1/4" wide copper foil with an
adhesive back, attached to the outside of the fuselage and covered with a
thin layer of fiberglass (not carbon fiber) to protect the foil. I'll
solder it together at the center and attach it somehow to the outside of the
BNC connector.

I'm planning to make four radials, each 22" long, connected at the center of
the antenna and oriented at 90° to each other.

I'm using narrow 1/4" copper foil for two reasons: 1) I already own it,
left over from my wife's stained glass hobby, and 2) since I'll be sticking
it on the outside, the narrow strips are more likely to stay attached to the
fuselage when covered with the fiberglass. I think a wider strip is more
likely to separate from the fuselage and create a bubble that would be
objectionable.

Here are my questions:

1. I'm planning to make each of the four radials from three 1/4" wide
strips run side-by-side, with 1/8" space between the strips. I hope that
this will provide epoxy bonding areas between the strips but still make the
antenna think that each radial is a single piece 1" wide. Any idea how the
performance of this might compare with a solid 1" wide strip?

Any differences will be immaterial.

The width of an individual ground-plane 'radiator element' is not particularly
significant. Lots of amateur radio stuff uses simple _wire_ for the radials.
Works just fine.

The only real concern, using 'foil' radials, is to ensure you've got enough
cross-sectional area in the radials for the transmitter power level.

2. Is there any benefit to soldering a foil strip across the ends of the
three individual strips making up each radial to bond them together at the
end opposite the center?

NO. There may, in fact, be a _slight_ disadvantage to doing so. At your
proposed 1/8" gap, it is, however, _VERY _UNLIKELY_ you'd see any measurable
degradation.

3. I'm assuming that 1" wide radials are significantly better than1/4" wide
radials; is that true?

'Better?', yes.
'Significantly', no.

Using more narrow strips, with the resulting smaller angle between them, is
better than a few wide strips.

Maybe I'd be just as well off to make each radial
out of a single 1/4" wide strip?

If you're going to run additional strips, run them as separate radials.

12 strips at 15 degree intervals will provide a closer imitation to a
true ground _plane_ than 4 somewhat wider strips will. With any 'non-solid'
ground-plane there will be some 'bias' favoring the direction
of each radial, vs. 'between the radials'.



4. The Com antenna is a Comant 122, which has a streamlined metal base a
couple of inches in diameter. Should the length of the ground plane radials
be 22" from the BNC connector at the center of the base or should it have
22" of length extending beyond the base?

the 'outside edges' of the radials should be the nominal 22" from the center
of the base-plate. Obviously, you can run them just from the outside edge
of the base-plate out to the required distance.

(Robert Bonomi)
shared these priceless pearls of wisdom:


-
-Here are my questions:
-
-1. I'm planning to make each of the four radials from three 1/4" wide
-strips run side-by-side, with 1/8" space between the strips. I hope that
-this will provide epoxy bonding areas between the strips but still make the
-antenna think that each radial is a single piece 1" wide. Any idea how the
-performance of this might compare with a solid 1" wide strip?
-
-Any differences will be immaterial.

That doesn't happen to be true. See next comment

-
-The width of an individual ground-plane 'radiator element' is not particularly
-significant. Lots of amateur radio stuff uses simple _wire_ for the radials.
-Works just fine.

That's because amateur radio stuff doesn't have to cover an instantaneous 15%
bandwidth ... 2 meters, for example, is just under 3% wide, and thin wire will
work just fine.

Having said that, and not having done the experiment, my GUT feeling is that
1/4" wide tape will be sufficient in and of itself, without going to the
3-strips-bonded-together trick for width. However, as in ALL fields, not doing
the experiment before welding the sucker together is cause for concern.


-
-The only real concern, using 'foil' radials, is to ensure you've got enough
-cross-sectional area in the radials for the transmitter power level.

Has not a WHIT of difference as to power level. You melt a one-mil thick
quarter-inch wide copper strip and you are running a flame thrower, not an
aircraft band transceiver limited by law to twenty-five watts.

-

-12 strips at 15 degree intervals will provide a closer imitation to a
-true ground _plane_ than 4 somewhat wider strips will. With any 'non-solid'
-ground-plane there will be some 'bias' favoring the direction
-of each radial, vs. 'between the radials'.

Not enough to measure. Try modeling it in EZNEC and see what you get.


Before you weld this sucker together, are you willing to temporarily tack-glue a
dipole INSIDE the fuselage and give it a try ... reporting your results to the
rest of us so that we can learn from your experiments? You *may* be able to get
away without making your pretty airplane look like a porcupine in heat.

I'll supply the materials for nuthin' if you will make the measurements I
outline to you.

Jim

Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

Hi Robert,

Thanks for your very thorough and thoughtful reply to my questions. You've
given me some good ideas to chew on.

Thanks,
Dennis Johnson