[TowerTalk] Mosley Antenna Question

[Jim Lux]

On 2/23/15 7:24 AM, Jim Thomson wrote:
> Date: Sun, 22 Feb 2015 19:20:45 -0700
> From: W0MU Mike Fatchett <w0mu at w0mu.com>
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] Mosley Antenna Question
>
> Tom,
>
> You hit the nail on the head.  Nobody said that XXXXX doesn't work.  I
> worked JA's on my Cantenna with 100 watts on 10m.  You can work people
> on anything.  I didn't pull my tower over in favor of stacked cantennas
> though....
>
> People were saying there could be better options.  I know the C3's I
> have worked much better than the TH7DXX.  Smaller, lighter etc... Do
> they both work.  You bet.
>
> Mike W0MU
>
> ##  A F-12 C3 is only 2 els on each band.   Imagine that, 2 full size els outperforming
> a TH7DXX...which has 3 els on each band, 4 on 10m, and a longer boom.
>
> ## This is all old news really.  2 els, full size, only has aprx 4.1 dbd gain. So much for
> traps.  A F-12 5ba  has 3 els on each band, and will make lunch meat  out of a trapped  yagi.
>
> ##  Baffles me why M2  doesn’t make any multi-monbanders like F-12, optibeam, and
> jk ants.

mfrs choose what they make for a variety of reasons.  M2 is happy doing 
what they do.


  Just thought of something else.  Putting a coil inside of an aluminum 
cylinder
> will decrease the inductance of any coil.

I'm not sure it actually changes the inductance.. The parasitic C in 
parallel can change the "end to end" reactance so that it looks like L 
is lower.

And, since the can forms a "shorted turn" (assuming it's not slit down 
the length), and that turn is coupled to the coil, the apparent 
inductance will change. probably lower, but it depends on the mutual 
inductance of the can and the coil (which might be quite high: the 
magnetic flux is fairly well coupled since the "turns" are concentric.



  So now you need more turns to get the same uh.
> Wind small gauge AL wire onto  a grooved polystyrene form, encased by an AL tube,
> and  you now have the worse possible config for a coil.

polystyrene isn't a great dielectric in terms of loss, but it won't 
change the inductance.  It will change the parasitic C (makes it 
bigger), and to the extent that the E field of that C interacts with the 
dielectric, there's increased loss.

Aluminum wire is an issue, but I think "small gauge" might be a more 
important one than the material.  After all, we don't copper or silver 
plate our HF antennas.

Skin depth in copper at 30MHz is 12 microns, 15 microns for Al (25% bigger)
The resistivity of aluminum is 50% higher than copper, so combining the 
increased skin depth and the increased skin depth, the net for Al vs Cu 
is that Al has 20% more loss than Cu (at 30 MHz)

Skin depth goes as sqrt(resistivity) and sqrt(1/f).. going lower in 
frequency doesn't change the overall calculation: the skin depth gets 
bigger, but the ratio stays the same.  At some point, when the wire get 
very small (<5 skin depth in diameter), then it's not skin depth any more.

AWG30 wire (pretty fine) is 0.01" in diameter, 254 microns, so at HF 
frequencies, pretty much any wire you use is going to be a "large" 
conductor, and resistance (or loss) goes as the 1/diameter, not 1/area.



  Q of the coil would be through the floor.
> Try that on a tank coil in a linear to see the effect.
>
> Jim  VE7RF
>
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