[TowerTalk] Coax

[Jim Thomson]

Date: Sun, 08 Apr 2012 13:17:06 -0700
From: Jim Brown <jim at audiosystemsgroup.com>
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.

http://audiosystemsgroup.com/publish.htm

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 A+B    

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 forever. 

later... Jim   VE7RF