[TowerTalk] 120 Film spools as spreaders
Jim Rhodes
rhodes@willinet.net
Sun, 07 May 2000 08:57:19 -0500
Another source of lightweight, inexpensive spreaders might be electric
fence posts made out of poly. The ones I have purchased for insulators in
the past have been labeled "fiberglass" but they look more like
polypropylene to me. Sold in various lengths & diameters at your local farm
supply store. Most round but I have seen some with a 'T' shaped cross section.
At 05:46 AM 05/07/2000, you wrote:
>In a message dated 05.05.00 16:09:36 Pacific Daylight Time, k3hx@juno.com
>writes:<<
> They survived 7 years without trouble outside. I was WA8MLV when I wrote
>the article in 1986. 73, Tim K3HX
> >>
> Sounds like they will last. Some 120 film spools are metal however.
>Based on 2 3/8" spacing of #12 wire, the Zo is 490 ohms and is a good working
>range. 6" spacing results in a 600 ohm Zo line. I have a commercial balun
>of 50-600 ohms made for rhombics so I'll be using the 600 ohm line.
>
>I examined the smooth plastic used in coat hangers someone suggested. It's
>flexible and strong. It's a bit small in diameter which limits its ability
>to hold the wire under stress with a hole in the end but I will give it a try
>if I can get a bunch of them. Small diameter is ideal from the standpont of
>low wind resistance and low surface area for dust and smoke to stick to. In
>the days of a lot of coal burning, it literally shorted out insulators,
>affected receiving and required cleaning. Bare copper wire even corroded.
>
>There is some rule of thumb of open wire spacing from metal objects like
>runing up the tower of being equal to its spacing. There is a simple test
>that will show that's not enough they never did. I use 1' minimum. When I
>GD open wire line with both ends shorted, I suspend it over the ground no
>less than 3'. If you let go lower the resonant frequency starts to change.
>I cut it to my magic multiple legth of 135.4' initially for 6" spacers every
>3'. Shorting the far end is ideal as it can be attached to a rope without
>affecting the resonant frquency. If 1/4 wave long, the Z at the open end is
>high and sensitive to what supports it. It can be supported by a spacing
>insulator if strong enough. More insulators will requires a few % of
>shortening as will insulated wire. Enameld is a minimum insulation. After
>installation and still shorted on both ends I GD it again and have to shorten
>it agaIn. I run it through wood walls with insulated filber sleeves in some
>cases. It likes total isolation but it all washes out in the Z at the end
>of the feedline. If it can be matched by the tuner no matter what it is,
>total transfer of power occurs there and to the antenna minus the losses in
>the feedline. The spacing can actually vary along the way unlike like
>coax--no big deal other can for looks.
>
>One little known benefit of 1/2 wave resonant lengths of open wire line is
>this. Assume the desired resonant length of the antenna is 3.562 MHz also
>for example purposes. With the feedline shorted, GD dip it there. If it
>doesn't GD there, trim antenna accordingly. It just assures the tuner sees a
>purely resistive load at the end off the feddline--for ease of matching. If
>it GD's at 3.8 MHZ, it doesn't necesary mean the antenna is resonant there.
>The feedline is no longer a 1/2 wave there--it's 8.5' too long and part of
>the resonant length. No big deal as the Z at the end of the feedline is
>purely resistive which is the major consideration.
>
>A little known trick is this I've used for 60 years that has never been
>written up--I will when I get time. Feed a 160/80/40 individual or parallel
>dipoles with a 1/2 wave multiple feedline and it repeats the antenna Z in the
>shack which can be close to 50 by height selection and or inverted vee
>bending. Even though this is a balanced feedline you can stick one lead
>right in the coax switch and the other to ground and it works great.
>Resonant each antenna low in the band, add a series BC variable Xc to ground
>in one wire and tune it right up the band. I'll resonate at 3.56 aprox and
>cover the whole band with a simple BC variable like no other system there is.
> If a 1:1 balun is used there, it would be better to have a variable BC
> Xc in
>each leg ganged together. Without the Xc, lengths of open wire line have to
>be removed for resonance higher in the band. I've had switches that do that
>for this and othe reasons. I've switched out or in a 1/4 WL of open wire
>line just to obtain the Hi or Lo Z I wanted for ease of matching or some
>other reason like direct feed into a Lo-Z link coupled tank circuit with a
>series Xc of years ago--or even a coax switch. There are all kinds of simple
>open wire tricks that assure maximum transfer of energy to and from the
>antenna unused today.
>
>With all due respect to Art Collins and his use of Pie/L networks designed
>for coax, he set ham radio behind and cost hams a lot of money for short
>lived coax for less RF to and from the antenna. Coax convenience is very
>costly and antennas with balanced feedpoints are still made which are still
>misfed causing feedline radiaton filling up Reflectors with all kinds of
>Mickey Mouse Baluns that don't work, affect the resonant frequency of the
>antenna and cause TVI. Gimme a break. Designsers still haven't got it
>right.
>
>I had 4 6M antennas using different feed systems in SD 2 years ago and each
>had TVI with 100W. When I used balanced 100 ohm coax or open wire line to
>feed a 2 element Killer Quad at 25' into a 275W MatchBox--NO TVI AND A GREAT
>SIGNAL. I could hold the frequency 50.125 MHz for 3 hour stretches. It
>worked so well I delayed the building of a linear. Unbalanced coax without a
>Bazooka (Collins did recommend that), Mickey Mouse Baluns that don't work,
>balanced feedpoint beams and the untuned mixers in so called modern radios
>have set ham radio back further than any other so called development. We
>need more advanced design and thinking--from the last Century. Stay tuned.
>I'll try and drag you back there for better ways to transfer RF. Open wire
>line is going to make a dramatic come back. k7gco
>
>--
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Jim Rhodes KC0XU
rhodes@willinet.net
--
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