Jim:
My suggestions embedded below.
73 de
Gene Smar AD3F
> I'm in the process of putting up an inverted L for 160. In the back
> corner of the lot I'm going to put up 30 ft of very skinny tower. The
> cross bracing looks like Rohn 25 but the face is only 6.5" The legs are
> solid. The whole thing is made of aluminum. I have 50 ft of the stuff
> but 50 ft would put it 20 ft higher than the HV line which runs down the
> other side of the back lane.
>
> The tower will be insulated from ground and fed at the base. To the top
> of the tower will be fastened a length of stranded Cu antenna wire which
> will run along the lot boundary to the top of a 30 ft mast (could dangle
> down a bit if I need the extra length) at the front corner of the lot,
> about 120 ft away.
>
> There will be a ground plate (we use plates here, not rods) buried at
> the base of the tower to which the coax shield will be connected.
>
> The tower will be self supported with maybe a couple of guys such that,
> if anything bad happens to it, it will fall inside my property.
>
> The 40 and 80 dipoles will be moved to this tower from the 16 ft mast to
> which they are currently attached.
>
> I have 2 questions
>
> 1. How to fasten the stranded wire to the top of the Al tower.
>
> I was thinking of just putting a stainless hose clamp around one of
> the tower legs, putting the wire through it and tightening.
> However, I think the wire would fatigue very quickly and break.
> Easy enough to get the tower down to fix it but would prefer not to
> have to do this every couple of months.
>
> Another thought was to wrap, say, 10 turns of the antenna wire
> around the tower leg and cover the bottom 5 or so turns with the
> hose clamp. These are the turns which are furthest from the wire to
> the other mast. The idea being that the wire could move more freely
> where it meets the tower and thus not be stressed at one particular
> point.
>
> Any other ideas?
Instead of tying the copper flattop portion directly to the tower and risking
fatiguing the wire by strong winds, you should consider terminating the flattop
on an insulator, leaving a long wire pigtail from the insulator to attach to
the tower. Tie off the insulator to the tower using appropriate cordage.
You should use a stainless steel band around the tower leg underneath the copper
flattop wire to prevent galvanic corrosion of the tower leg. Then apply
a conventional stainless hose clamp around this entire assembly, then a layer
of
3M Type 88 electrical tape to keep the water out. If you use the tape, make
sure the flattop wire pigtail enters the hose clamp from underneath, not on
top, forming
a drip loop.
>
>
> 2. How to deal with radials
>
> Not much room for radials. I can run one about 120 ft along or
> under the ground right underneath the flat top. I can maybe mickey
> mouse a few more about 20 ft long and that's about it. The fact
> that I may have ground losses of 3 dB or so doesn't really trouble
> me as almost no matter what I do it will be better than using the
> 80m dipole on 160. What does bother me is that about the only
> sensible place to run the coax feed is right along the 120 ft
> radial. I presume, then, that I'll have mucho current on the
> outside of the shield and tons of RF in the house and shack.
>
> I'm proposing to wind a coax choke at the base of the tower and
> another one where the coax enters the house in hopes of suppressing
> this shield current.
> Any comments on the proposed installation would be greatly appreciated.
>
That ought to work. Just make certain your choke presents sufficient
inductance
at 1.8 MHz or so to be effective. Enough inductance? I've read on these pages
that 500 Ohms (10X the coax impedance) should be the target. You can use your
antenna analyzer to measure the shield's impedance. Connect one end of the
choke's
shield to the analyzer's center contact, the opposite end of the shield to the
ground
contact.
> Lightning isn't an issue here, thank goodness.
Lucky man!
>
> 73 and TIA for your help. See you (well, those few of you who will be
> able to hear me) on 160.
>
> Jim Smith VE7FO
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
> Weather
> Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
> and ask for Sherman, W2FLA.
>
> _______________________________________________
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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