Date: Sun, 08 Apr 2012 13:17:06 -0700
From: Jim Brown <firstname.lastname@example.org>
Subject: Re: [TowerTalk] Coax
On 4/8/2012 10:46 AM, Bob Ad5vj wrote:
> I bought off Ebay 100' of '400' 50ohm coax. Thinking it was LMR-400.
> This has got to be the stiffiest, lightest weight, cheapest connector, crap i
> have laid my hands on.
> Where can i buy REAL LMR400 coax?
There's nothing magic about LMR400 that justifies its cost for operation
on the HF bands. For use below 100 MHz, Davis 213 is as good, of good
quality, and a lot less expensive.
For more about coax, see the Q&A tutorial that's on my website.
73, Jim Brown K9YC
## say what. Run 200 feet of 213-U into a dummy load at the far end, with
a wattmeter 1 foot before the dummy load.
Then repeat the test, with LMR-400 or LMR-400UF. Stuff a 1 kw CXR into
both of them on 29 mhz, then see how
much higher the wattmeter is with the LMR-400. Then calculate the loss.
## To do that test correctly, you require a 2nd wattmeter, on the output of
the amp. They don’t even have to dead on accurate either, as long
as they BOTH read the same, when installed nose to tail, a few inchs apart.
IE: hook 2 x wattmeters nose to tail on output of amp....then into
a dummy load. Apply 1 kw cxr, and see what each one reads. If one reads
1000w..and the 2nd meter reads say 965w ..or 1045w, then
note all that down....and factor it in when doing your calcs. Also note which
wattmeter gets installed at the other end of the coax.
## If using bird line sections, you can use just one slug..and move it from
one line section to the other.
## I know, there is 20 other ways to calculate line loss, but the above is the
real deal. A buddy once had just over 300 feet of 213 u
on his stacked 10m array. With 1 kw applied in the shack, he got 335 watts
at the top of the 100 ft tall tower. 213 was replaced with
.875 inch heliax. Power went up to well over 900w at top of the tower. That
was a real eye opener.
## here’s what Times microwave shows on their on line calculator..for 100
foot of each type of cable @ 29 mhz.
LMR-1200DB .205 db
LMR-600db .414 db
LMR-600UF .501 db
LMR-400db .666 db
LMR-400UF .799 db
RG-213U 1.024 db
And the above table is for only 100 feet. If you have say 140 feet, then
multiply by 1.4 etc. With 200 feet, your
losses on 10m and above are sky high. Dunno about you folks, but I measure
the power at the ant, and also at the back
of the amp. You end up with 3 x choices. (a) increase the power output,
and-or replace amp with a much bigger one,
like double the capacity. (B) obtain much bigger coax. C use a combo of
To say there in no difference between 213 u and LMR-400 below 100 mhz is a
bit of a stretch. Not everyone can install a 100’ tower
right out their back door either. By the time you feed coax through basement
or attic, crawl spaces, routing around the outside
of a home,garage, etc, you have gone through a fair chunk already. Then over
the base of the tower...which may well be 50-150’ away, then
up 100’. 213-U..... no I don’t think so. Sigs lost in the coax are lost
later... Jim VE7RF
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