[Boatanchors] Slightly, OT: MF-HF Coax cable loss with high SWR (long post)

Thu Sep 22 18:18:08 EDT 2011

I wouldn't cut the 213 either, but rather, roll it up and leave it outside, 
maybe under the trailer and then run the bitter end inside through a 
grounded feedthrough.
Just my 2  cents.

I used 213 exclusively aboard ship (oceanographic research) for all my 
receiving and transmitting to inverted "v" and long wires.  The inverted "v" 
was RG-58 with the shield peeled back and folded as 1 leg of the V.
They (the cables) were not routinely kept out of the weather and salt spray 
and still gave me no trouble as long as I made good connections with type N 
connectors and water proofed the connections as if the wire was going in the 
ocean.  The only trouble I had was with the foam filled cheap cable.  It was 
less loss for my satellite work but water migrated into the foam and caused 
much trouble.

Ray

-----Original Message----- 
From: Phil KO6BB
Sent: Thursday, September 22, 2011 2:53 PM
To: Boatanchor List
Subject: [Boatanchors] Slightly,OT: MF-HF Coax cable loss with high SWR 
(long post)

Hi All,
Over the past 2-3 years the RFI in this mobile home park has gone up
sharply and is MUCH higher than when we moved in 11 years ago.  Because
of that, combined with the poor propagation due to sunspots it got so
that I wasn't doing any HF ham radio operating at all, though I have
remained active in SWLing (higher power stations :) Because of that,
along with other reasons I sold my Yaesu FT-2000 and just SWL
now-a-days.  That combined with other interests taking my time, I just
couldn't justify that expensive of a radio to just sit there receiving
almost no use in the past two years, when our bank account was nearly
empty.

Anyway,  my primary antenna has been an 88 foot long dipole fed with
ladderline as it was by far the best performing multiband antenna I've
used here for hamming (with a tuner of course).  I've tried a lot of
different skyhooks, but the 88 footer worked best.  Verticals over this
sheetmetal roof worked VERY well also, but had a very high noise level.
With almost no yard space, all my antennas must mount over the mobile
home roof.  You can see the present dipole in this picture (the
Butternut HF-2V is long gone).
http://ko6bb1.multiply.com/photos/album/25/New_wire_antenna_Field-Day_2009_from_Home#photo=6

In doing some experimenting with the 88 footer I determined that a lot
of the local noise is being picked up on the ladderline.  Yes it's
twisted, but unfortunately, and in this installation it can't be
avoided, it runs right over the electrical box with all the circuit
breakers, incoming/outgoing wiring to the box etc (and this mobile home
doesn't have metal conduit for shielding).  Climbing up on the roof with
a portable radio and the ladderline momentarily shortened to just reach
the radio, the noise level dropped a great deal, while signal strength
of WWV at 5, 10 and 15 MHz seemed to remain about the same.  Voila,
noise coming from the wiring in the walls.  In disconnecting TV's,
computers and nearly everything else in the house and walking around
outside with the portable, most if not all of the noise is actually
coming in on my powerline, not generated by my own stuff.

Taking the same portable and walking around with it, then going up on my
own roof the noise level is MUCH lower up there, so I suspect the
sheetmetal roof acts as a shield, at least to some extent.

Anyway, I said all that to say this.  Since I am no longer transmitter
on the HF bands (still do VHF/UHF), I've decided to try one of the low
noise antennas designed for receive only.  I've always been skeptical of
them as I figure a "wire is a wire".

An exception seems to be the Par EF-SWL antenna.  See it here. . .
http://www.eham.net/reviews/detail/3707

NOT a radical new design, and I understand the principals behind it of
using a separate isolated ground for the antenna and transformer
coupling to the coax.  The principal works very well for people in the
LF/VLF hobbies (I used to do a LOT of LF beacon work).  I "COULD" make a
9:1 coupling transformer and build my own much cheaper, but I'd have to
locate a source for the binocular ferrite core, waterproof housing,
etc.  I DO have a series of ground rods tied together for the station
ground, and it will be no problem to disconnect one and use it for the
antenna.  It's even right at the base of the push-up pole that I'd use
to hold the coupler end of the antenna.

While it hasn't arrived yet, I have every reason to expect that it will
prove to be an antenna with as low of a noise floor that I can install
at this location.

NOW THE QUESTION.  The antenna is Coax fed, the coax run will only be
about 30 feet, give or take. Normally at MW and HF, for receive only I
wouldn't worry too much about "coax "loss" on that short of a run.  I
have a 55 foot length of nice Mil-Spec RG-213 with PL-259's on each
end.  I also have a nice long length of Mil-Spec RG-58 with no
connectors that I could cut to length and use.  I'd rather not cut the
RG-213 in case I want to add a higher VHF/UHF antenna in the future.

So, the RG-58 would be my first choice for "receive only" at HF.  My
only concern is that I realize that this antenna is going to have
extremely wide SWR swings, and thus have SWR-loss across the 1-30MHz
spectrum.  Is this loss on a 30 foot run likely to be significant enough
to be of concern across the MW-shortwave spectrum, or should I just go
ahead and cut the RG-213 to length and use that (it's too stiff to roll
up and shove under the overhead shelf)?

Thanks all. . .

-- 
73 de Phil,  KO6BB
http://ko6bb1.multiply.com/ (OTR Blog)
http://www.qsl.net/ko6bb/  (Web Page)

RADIOS:
Grundigs: Satellit 750 (2011), S-350 (2006)&  G6 (2011).
Radio Shack: DX-380 digital portable (circa 1990).
Yaesu: Two FT-8800 Mobiles, VX8R Quad-Band (circa 2010).
Zenith: Royal-7000 Transoceanic (circa 1969).

ACCESSORIES: MFJ 1040C Pre-Selector, W9GR Audio DSP.
ANTENNA: 88' Long balanced dipole.

Central California

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