[TowerTalk] Solid 160 radial plates?
jimlux at earthlink.net
Fri Aug 24 11:10:03 EDT 2018
On 8/24/18 7:21 AM, Chick Allen via TowerTalk wrote:
> I've been reading a lot of older threads about using ground shields (mesh or solid copper ground radial plates) at the base of an Inverted L for 160. Several contend that AM broadcast stations use copper shielding at the base feedpoint with radials attached around the perimeter as currents are max there. Even the length of the radials becomes a question as one thread says the velocity factor of radials on the ground is 0.50 and the at the proper radial length, for on ground/slightly buried radials, is more like 80 feet and not one quarter wavelength. Buried radials do not need to be resonant so my question centers more on the use of a solid, or mesh, material at the base. Comments on this practice please?
The difference, if any, would be in the RF resistance of the plate,
whether solid or mesh. I would think it wouldn't make a huge difference.
A mesh has more possibility of corrosion at the contact points in the
One thing to think about is that skin depth in copper at 1.8 MHz is
about 2 mils. if you have a conductor thicker than 5-10 skin depths,
there's not much current flowing in the middle of the conductor.
A 1/4" thick plate (250 mils) is really only carrying RF current in the
outer 10 mils of each surface, so 92% of the copper is just there for
structure and beauty.
For reference and comparison:
10 mils is AWG 30, which is pretty fine wire.
1 oz copper clad is 1.4 mils thick.
- I'd worry about mechanical issues and how you're going to attach all
those wires, more than whether it's solid or mesh. Copper sheet isn't
much better than aluminum, from a resistance standpoint. However, with
copper, you don't have the dissimilar metals issue, and you can solder
to it with a (biggish) torch (it's hard to get a big sheet of copper hot)
Realistically, what might be easiest is just bundling bunches of the
ground wires together, solder or braze them in a batch to something like
a AWG 10-14 wire, then bunch all the big wires, and solder those. This
is something that is easily done with a propane torch, and while it
won't be as pretty as a shining polished copper plate, its RF properties
will probably be superior.
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