Ive been using that CATV RG-11 since the late 70's as jumpers to the
hardline from wire antennas, in the house, and as phasing lines for 160 and
80M verticals. Never a hint of heat at 1200W but someone with a 3CX15000
might have a different opinion. Measured loss at 2 MHz was .17dB/100'
Several lengths are over 35 years old, reused several times MF to UHF, and
pass regular sweep tests.
Carl
KM1H
----- Original Message -----
From: <mstangelo@comcast.net>
To: "Donald Chester" <k4kyv@hotmail.com>
Cc: <topband@contesting.com>
Sent: Friday, June 13, 2014 3:49 PM
Subject: Re: Topband: 50 ohm direct burial coax cable
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
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