[TowerTalk] Top loading mobile antennas

[John Tait]

Hi Phil..

    Mark modestly left out his web address which is at
http://www.qsl.net/n1lo/index.html
It's well worth a visit, and not only for his mobile info. I use homebrew 
whips similar to his,
with stiff copper wire for my capacity hats.
73
   John EI7BA

   >
> Anyone ventured there yet in attempting to improve the operation of the
> Hustler on 80 meters?  If so, I'm interested in the size rods used, any
> problems encountered, etc. Thanks,  73   Phil / K4MPE >>
>
> Hi Phil,
>
> I have been using homebrew mobile HF antennas for just over 5 years. The
> most efficient way to load a shortened antennas is top loading, typically
> achieved with capacity hats in various configurations. It's interesting to
> see that a commercial product has come out for just this purpose.
> Top loading of fixed verticals can also be accomplished buy adding guys to
> the top of the antenna. Starting at the top, the guys are wire, terminated
> in insulators, then rope. You'll have to modify the resonator to remove 
> coil
> turns to re-resonate the antenna where tou want it.
>
> If I remember correctly, the Hod Rodz are a set of spoke rods, with no
> perimeter wire, with a little hub that clamps onto the whip.
>
> For mobiles, this requires a much stronger whip, but the cap hats are
> extremely effective in reducing the amount of coil (and losses) required 
> to
> load a short whip. The closer to the tip of the whip the loading, the 
> higher
> the efficiency. However, they can't handle trees too well!
>
> I have made lightweight, spiderweb-looking, cap hats from stainless wire
> that were effective but just not practical. I'm working up a new design 
> that
> is much more practical for mobile. I hope to have a new plan out on my
> website soon to share the construction details.
>
> The idea is to use a single, trailing whip at the top. Electrically, it's 
> an
> inverted 'L'. Since there is no ring/perimeter wire, and no other spokes, 
> it
> handles limbs nicely. The mechanical details are the rub.
>
> The antennas I'm making now are homebrew inverted L bugcatchers. However,
> the trailing top load would benefit a screwdriver antenna just as
> effectively as a clip on accessory.
> I'm using 6-7' long fishing rod cores, which are extremely light and 
> stiff.
> Since they point to the rear, they don't catch limbs.
>
> Just be careful when backing up near any limbs! You guys out in open land
> don't need to worry, but here in VA, there's lots of potential limbs to 
> hit.
>
> The vertical section of the antenna is about 10 feet, and the trailing 'L'
> section is about 8 feet. With the coil bypassed completely, the antenna
> resonates on 20m.
> It is possible to also electrically disconnect the top load using a 
> jumper,
> without physically removing it from the antenna, allowing you to tune the
> higher bands as normal.
>
> Having the L section greatly reduced the amount of coil turns needed to
> resonate the antenna, especially on 80m. This also tends to create more
> vertical skywave radiation, and I have finally been able to consistently
> work the VA Phone net on 3947 from the mobile.
>
> To support such a top 'L' load, I use the combination of a stiff whip and 
> a
> guy line. The whip is a section of 1/2" solid fiberglass, with a spiral of
> wide, tinned braid on it, and taped over. Imagine a Hamstick, but with 
> 1/2"
> wide braid on it. BTW, I'm using 1" copper water pipe for the base mast.
>
> I'll save the mechanical details for the plan, but you get the idea. You 
> can
> make a clip that holds a commonly available Wilson 1000 CB whip at the top
> for an 'L' trailing top load.
> I have a pic of the mobile. If interested, send me an email and I'll send
> you a copy. I'm still experimenting with the details and it will be a 
> little
> while before I post the plan on the web site.
> I also have a jig now, which makes it much easier to wind air-core coil
> stock.
> Happy Building!
>
> --...MARK_N1LO...--