Why Multi-band Dipoles Need HEAVY Open Wire Line
by Tom, K1JJ
|So Tom, what's the deal with the
"Brown plasdick type" twinlead? What's wrong with it, is that a specific
brand or style, and what do you suggest otherwiise? I've got a bit of 400
ohm (brown) twinlead from the wireman, and if I could do better with
something else I'd rather start that way. My time is what's at a premium
here so I need to listen to the voice of experience and not duplicate less
than optimum moves if I can help it.
Well, the brown plastic open wire I mentioned is the common stuff seen with the punched square slots... about 2" spacing and about 450 ohms. Maybe #18 wire or so?
That stuff is OK as long as it has a "reasonable" match (reasonable swr) so that the feedline current points are not too high. It's just a matter of how much loss you consider too much.
For example, for a 40M dipole of the 3 three wire type or a folded dipole type, the input is about 600 or 300 ohms respectively. Using that 450 ohm line would be excellent and the loss wud be nil on 40M. It is matched correctly.
However, using say, a standard 40M 1/2 wave dipole on 160M, the input is like 5 ohms and the swr is about 10:1. That brown feedline would probably melt at the current peaks running a KW thru it for sure. Big losses. Axe Tim-Tron about his own melting experiences with this stuff.
Running this same 40M dipole on 75M would produce maybe 12 ohms, so the swr is now about 5:1 - you could probably get away with that. This LOW impedance is the killer here.
But most dipoles cut for a band are about 70 ohms... fed with 450 ohm line is about a 6:1 swr ratio. This is commonly done. I imagine when the swr gets much above this figure is when ya need to worry about it for that plastic line.
And so forth as you use a 75M dipole fed with this plastic open line on the higher bands. There will always be a place where the swr is very high, even if the antenna is MUCH longer than a 1/2 wave for the band. On some freqs you will luck out and be close.
Chuck, K1KW once ran a test wid that stuff. He terminated 100' with a 450 ohm resistor dummy load and put in 100W on 14 Mhz. (Probably used an ant tuner) He measured 98W coming out the other end. So, if matched, it works VERY FB.
In contrast, take some homemade openwire, say #12 insulated wire from Home Depot spaced 5"-6" with good Lexan or Teflon spacers. The IR losses are MUCH lower for both the wire size and for the better insulation. You can then tolerate a higher SWR that results from running any dipole on bands with high swr. The bottom line is that every 1/4 wave along the feedline you have a current peak or a voltage peak. This "standing wave" magnitude varies from none when matched, to wild swings when the swr is high. The radical current peaks are the problem and cause heat and loss.... even in open wire.
The DB loss is always the same, no matter what power level you run. So running LOW power with lossy feedline is not going to help the situation.
Hope this helps. The bottom line is that loss is relative, and it depends upon your own anal/perfectionist factor, the power you run before it melts, and whether or not you can run BIG open wire for XYL-approved reasons...
Personally, I run hardline and coax for ALL antennas here, simply for convenience. There was a time when I had five antenna tuners mounted on the wall for fast band switching. There's really little on the air difference in the end if losses are kept in check with any feedline method used..