Back To The Drawing Board

Some days you eat the bear and some days the bear eats you.  Success is going from failure to failure with enthusiasm.  I know. You’ve heard ’em all.  This was one of those days.  Time to redesign.   When the pole was up to about 45 degrees one of the fiberglass sections snapped.  As it came crashing down around me two more sections snapped when they hit the ground.  Three sections are broken and replacements are not available on ebay.  The picture below is NOT how it’s supposed to look.

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The ladder was being used as a gyn pole raising fixture and it did it’s job very well.  Fiberglass camo poles just aren’t strong enough to lift their own weight when they’re longer than 8 sections.

Aluminum sections are the only type I could currently find on the market forcing a switch.  Ladder line won’t work inside aluminum and that dictates we switch to coax.  We’ll need a balun at the top.  More weight to raise.  Otherwise the design will stay the same. Aluminum poles are on order.  Hopefully aluminum sections will be strong enough. Work is on hold til they arrive.  Meanwhile…

Thursday, April 14, 2016 Update –  I’m having second thoughts about switching to aluminum and coax.  I’d be much happier if I could figure out how to get the original design working.  Today I found a company on the Internet that says they have fiberglass army poles.  I think fiberglass will work if I lift the pole one section at a time from the bottom like a telescoping pole.  I’ll make the mounting post taller so the top clamp holds the pole at about 5 feet above ground.  Then I can slip in the next section and raise the pole.  Repeat until the proper height is reached.  I will loosen the guys enough each time to allow raising the pole one section but not enough to allow the pole to fall over.  I tried this method for the remaining unbroken poles in the picture below and it seems to work.  It only needs 3 more sections inserted at the bottom.

Oops. I called to make sure the company that advertised fiberglass poles really had them.  They do but the shipping is $48.00.  I passed.  (After about passing out.)  Still hoping for fiberglass, I will try to repair what I can and reuse the broken sections.  Lonely and waiting for attention the half done support pole awaits.

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The article below might be one reason why I had second thoughts about going to coax.  This is stolen from KV5R’s web site which I highly encourage one to visit:   http://kv5r.com/ham-radio/ladder-line/

 

Ladder Line

Copyright © 2002-2015 by Harold Melton, KV5R. All Rights Reserved.

Why Use Ladder-Line?

To efficiently feed a non-resonant multi-band antenna.

Antenna Mythology

Resonance

First, let’s dispel the greatest myth in antenna theory: Antennas must be “resonant” to be efficient. Baloney! It just ain’t so!

Please recognize that an antenna need not be resonant in order to be an effective radiator. There is in fact nothing magic about having a resonant antenna, provided of course that you can devise some efficient means to feed the antenna. Many amateurs use non-resonant (even random-length) antennas fed with open-wire transmission lines and antenna tuners. They radiate signals just as well as those using coaxial cable and resonant antennas, and as a bonus they usually can use these antenna systems on multiple frequency bands.

ARRL Antenna Book, Ch. 2

As long as the length of the antenna is at least a half-wavelength at its lowest intended frequency, its efficiency is well over 90%, just like a resonant dipole. The problem is getting power to it—coax is very lossy (due to dielectric heating) unless terminated into its characteristic impedance, and this effect is what leads most hams to erroneously believe that non-resonant antennas are inefficient. But the problem isn’t non-resonance, it’s high SWR on coax.

On the other hand, ladder-line does not suffer from high losses at high SWR, so may be effectively used to feed an antenna that may, at various frequencies, present the feed-line with any SWR from 1:1 to ~10:1. So, with ladder-line, you can completely forget about resonance and SWR, until you get to the radio, where you use a tuner to make the match to 50 j0 ohms.

To compare mismatched feed-line losses we have to start with the antenna’s feed-point impedance, and the line’s impedance, then calculate the SWR, and finally, the loss of each feed-line-type at a given frequency and length.

For a worst-case example, feeding a voltage node (like running 40 meters on an 80 meter dipole), let’s say the feed-point impedance is 3500 ohms. With 100 feet of RG-8 coax at 7 MHz, that’s a whopping 65:1 SWR, with a total loss of 78%. With 600-ohm open-wire line, the SWR is only 5.8, and the loss is 3%! Then, if we switch to 80 meters, the impedance is 50 ohms, the SWR is ~12:1, and the loss is 7%. In this case, 450-ohm line would be even better, because the SWR only varies from about 9:1 at 50 ohms to 7.7:1 at 3500 ohms. The total losses for 100 feet of 450-ohm windowed ladder-line, at 9:1 SWR, ranges from 5% at 3.5 MHz, to 14% at 28 MHz, and again, that’s at the worst-case mismatch points.

So we see that ladder-line is not only better for non-resonant antennas because of its much lower loss at high SWR, but also because its characteristic impedance places it nearer the center of the antenna’s impedance range, from lowest (odd half-waves) to highest (even half-waves).

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