Don,
You bring up some good points. I got a couple of hundred feet of Flooded
Commscope 75 ohm "RG-11" type at a good proce. I don't have the part number
handy. I've been very happy with it at 100 watts. I'm thinking of running 500
watts. The center conductor is copper clad steel. I'm concerned about RF
current losses because of the skin effect on 160 meters. Does anyone have any
experience with this type of CATV cable at high power?
A 1:5 maimatch may be an issue at the amplifier but I guess I can place a L
matching circuit between the amp and coax to get the SWR down.
I like Flooded cable not because it is anti-fungal but to keep out moisture. My
coax runs 45 feet underground and 20 feet under my deck. I had some groundhogs
visiting us under the deck and fortunately they didn't have a taste for the
coax. The visitors have been relocated.
Mike N2MS
----- Original Message -----
From: Donald Chester <k4kyv@hotmail.com>
To: topband@contesting.com
Sent: Fri, 13 Jun 2014 19:17:59 -0000 (UTC)
Subject: Re: Topband: 50 ohm direct burial coax cable
I think you're being overly picky. The 75-ohm cable would be at worst a 1.5:1
mismatch. Probably a fraction of a dB loss at most, particularly with foam type
hard line. If you have a source of good, low-loss 75-ohm coax that will safely
handle the power you intend to run, and which will hold up for an extended
period under direct burial, use it. The most likely consequence might be a
slight re-adjustment of your matching network.
>From my experience, the greatest problem even with direct burial cable has
>been rodents chewing on the plastic outer jacket, but moisture infiltration
>and corrosion may be a problem as well. I am not aware of any fungal problem.
>I started out with a 140-foot run of RG-213 direct burial rated cable to the
>base of my series-fed 160m vertical. When I first tested with fresh coax, it
>was running about 93% efficiency to a dummy load at the far end of the
>transmission line. After about a decade I re-tested it using identical
>measuring procedure, and the efficiency had dropped to slightly less than 80%.
>I replaced the cable and once again the reading came up to the original 93%
>efficiency, but after only three years it had dropped to around 83%. The
>second time I didn't find any evidence of rodent damage as in the previous
>case (however I didn't pull up and inspect the entire length), but I ended up
>erecting poles, installing fresh cable, and running it elevated, 8' off the
>ground.
I no longer use coax with this antenna at all. I eventually replaced it with a
run of 440-ohm balanced open wire line, #8 copperweld conductors spaced 2 1/2"
apart, elevated using the same poles. Comparing rf ampere readings at the base
feed point using a thermocouple meter, with the same DC input to the final, the
reading was noticeably higher at 1800 kHz with the OWL system, and about the
same or just slightly higher at 2000 kHz. These are overall efficiency readings
that include likely variations in losses with the different types of matching
networks used at each end of the two types of transmission lines.
Don k4kyv
-----Original Message-----
From: mstangelo@comcast.net
Does anyone know of a source for 50 ohm "RG-8" type direct burial coax able.
I can find many sources for CATV and Satellite 75 ohn cable but not 50 ohm.
Direct burial cable is coated with and anti-fungal compound. I''ve had
success with 75 ohm cables for receiving and want to purchase 50 ohm direct
burial for transmitter feeds.
Thanks,
Mike N2MS
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|