Stephen Reichlyn wrote:
>
> Well, after many stops and starts, I have finally gotten a 160M inverted L
> up on the island. I started with an inverted L for 40-80-160 using Unadilla
> traps. The 40M section worked great but the 80 and 160 sections wouldn't
> take more than 500W without breaking down. Rather than fool around with that
> configuration, I took it down and tried something else on Saturday.
>
> The new antenna is constructed from #16 insulated wire (1/4 wavelength). I
> have mounted a 40 foot portable aluminum mast at the end of my dock, mounted
> to a piling, 150 feet out into the salt water. The end of the L slopes
> gently down to about 20 feet above the water at high tide and 27 feet above
> the water at low tide. I have constructed a simple ground system by sinking
> a 10 foot ground rod into the ocean floor under the dock. I have attached 4
> 125 foot radials to the ground rod which are spread out in the water. The
> feedpoint of the antenna is attached to the ground system using heavy
> tinned-copper braid. The radials and rod are covered with salt water all the
> time.
>
> The dock is about 15 feet above the ocean floor at its end. The floor gently
> slopes into shore. The tide is about 6-7 feet difference from low to high.
> After some testing on Saturday night and last night, I find the following:
>
> The antenna transmits and receives VERY very well. Lots of EU, AF, SA and NA
> worked with good reports.
>
> At low water, the antenna SWR is 1:1 at 1830 (using an Amidon UnUn 50:12.5
> ohms) and the antenna shows about 40 kHz of bandwidth (2:1 points). At high
> tide the resonant point shifts up to 1860 (same feed system, same antenna
> resistance, same bandwidth).
>
> My question is: Do you think there would be less resonance change if the
> horizontal portion of the L was really horizontal (I could probably get that
> part of the antenna up to 35-40 feet, the mast/support end is probably 50
> feet above the water at mid-tide. Please reply direct if you have any advice
> or comments. Thanks 73 Steve AA4V
>
I find it interesting that the resonant frequency INCREASED. I would
have expected that, at high tide, there would be an increase in capac
itance to ground--the water--and a lowering of resonance. I would be
pleased to learn why that is not so.
At any rate, an antenna like that is a flat-out wet dream. I am consumed
with jealousy!
--
Garry Shapiro, NI6T
Editor, The DXer
newsletter of the Northern California DX Club
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