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Author Topic: coax  (Read 739 times)
KD5MMJ
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Posts: 21




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« on: November 25, 2001, 03:16:48 PM »

I need advice for coax.  My main operating is going to be cw in the 40m band, all QRP.  In fact, most of it will be will be in the QRPp range.  Location is one story house and i seriously doubt that length of feedline should be greater than 50 feet long. It's an estimate but it should be even shorter than 50 feet, maybe around 25-30 feet. What fifty ohm coax should i get? Should i get a special low loss type or LMR or Belden "specialty" type coax?
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WB2WIK
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Posts: 20574




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« Reply #1 on: November 26, 2001, 04:19:38 PM »

For such a short length, especially at 7 MHz, you won't notice any difference between small, flexible cables such as RG58/U or RG8X ("mini-8") and larger, inflexible cables such as LMR400 or 9913.  Less than 50 feet at 7 MHz, in any of these cables, has essentially no loss at all.  In fact, "zip cord" commonly used for wiring lamps and such is excellent at this frequency, and in common use, with almost immeasurably low loss.

The specialty cables such as LMR, 9913, etc, come into their own at VHF-UHF frequencies (typically above 144 MHz), and especially when longer lengths (more than 50 feet) are needed.  At 7 MHz, nearly all loss in the transmission line is Ohmic, meaning caused by the resistance (which is based purely on the circumference of the conductors used) of the conductors and not by other factors such as dielectric loss and shielding efficiencies.

When working QRP and QRPp, surely you want to minimize all losses as much as possible, but the coaxial cable, especially at 7 MHz, just isn't one of the lossy elements.  Antenna efficiency plays a major role, as the simple difference between a dipole installed at 20' above ground vs. the same dipole installed at 70' above ground is enormous.  I'd concentrate on the antenna(s), good headphones, having a very clean (transmitted) signal, operating when the band is in the best condition (typically during the dark hours, and peaking at grayline) and lots of operating skill to get the most enjoyment from 7 MHz QRP work.

73 de Steve, WB2WIK/6
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N8MHD
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Posts: 11




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« Reply #2 on: November 26, 2001, 05:47:54 PM »

Check out the Cable X-Perts web page, www.cablexperts.com (coax, HF group).  They give loss values per 100 ft at a variety of frequencies, as well as prices.  (I've bought stuff from their web site, and service was good).  For 50 feet of cheap coax, you're looking at about .25 db loss from RG8U, vs. .5 db from RG58 (or higher, depending on the variety).  Not much, but noticeable.  I got their cheap RG8U for running down from my attic, and it is some effort to work with -- not bad, but somewhat awkward.  If the loss doesn't bother you, or if the price makes a difference, get the RG58.  Maybe put up more expensive cable second round.  After all, you don't ever want to be done with it all.

Peter
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KE3HO
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Posts: 235




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« Reply #3 on: July 01, 2002, 12:19:01 PM »

The first two replies answer the question well. I will add just one thing. The manufacturer's stated losses per 100' for coax are only true for 1:1 SWR. If your SWR is high the losses will be higher than published values. Make sure your antenna is a good match to the coax. Note also that using a tuner does not change the SWR in the coax, it only changes the SWR seen by the rig.

73, Jim
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