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Re: [TowerTalk] Coax Question

To: "Tower and HF antenna construction topics." <>
Subject: Re: [TowerTalk] Coax Question
From: jimlux <>
Reply-to: "Tower and HF antenna construction topics." <>
Date: Wed, 14 Apr 2010 06:47:15 -0700
List-post: <">>
Jim Thomson wrote:
> Date: Tue, 13 Apr 2010 21:53:49 -0700
> From: jimlux <>
> Subject: Re: [TowerTalk] Coax Question
> Kipton Moravec wrote:
>> On Tue, 2010-04-13 at 07:15 +0000, van wd8aam wrote:
>>> In regards to long runs of coax, has any thought EVER been
>>> given to RG213 to a remote tuned high power amplifier
>>> at the base of the tower.  I've thought about it, and for
>>> costs, it might be cost effective.  I'm sure someone has this
>>> working for them somewhere.  What do you guru's say to this?
>>> van
>>> wd8aam
> ##  it's already been done.. and that was 10 yrs ago ! 
> The kicker is... if you have say 3 db of loss back to the
> station, then you need an extra 3db of drive power. You 
> also LOSE  3 db on RX !    You also require a LONG run of  240 vac
> line, to base of tower,   OR a 240-600 vac  xfmr  at the house..
>  then  another  600-240vac step down xfmr at base of tower. 

Or a run of the mill 240VAC line and a boost/buck transformer to make up 
the voltage drop at the load end.

There are other reasons why one might want a back to back transformer 
approach (galvanic isolation).  I don't know that I'd want to run a 
600VAC feeder in a residential environment.

> ##  getting the power out there is gonna  kill it right there. 
> 600-240  xfmrs  [can be run either way]   in 5-10 kva 
> variety are available surplus.. then  some small ga wire
> could be used.  Usually, the xfmr at the far end will have
> taps on it.. like 575v.. or 550v etc.... to factor in the V drop,
> so u end up getting the full 240 vac... but only under full bore load.
> No load, the V will rise > 240vac .  BIG  ga  AL wire between the
> 2 x xfmr's would work. 

You have power loss either at line frequency in AC wires (cheap to make 
up) or at RF in coax (expensive).. Every watt you put out in RF takes 2 
or more watts of AC power to make, eh?

Run normal wire (12ga), let it dissipate the power, and make up the 
voltage with a boost transformer at the far end.  Let's run some numbers..
500 feet of AWG12 *2 = 1000 ft = 1.6 ohms.  You're going to be drawing, 
say, 10 amps (2400W), so you have 16V of drop.  First, 16V is only a 7% 
voltage drop.  If the panel end is at 240, you're still above 220 at the 
load, and your power supply might have a tap for lower line voltage 
(e.g. 208V).

But, if you really want to get back to 240V.. a 15Amp boost transformer 
for 16V is only 240VA.. (it's a 240V primary, 16V secondary 
transformer).. that's a fairly inexpensive item and fairly small (think 
of a landscape lighting transformer, which could be used for this).

Something you could even consider is running even smaller wire (AWG14, 
AWG16, etc.) and using more boost.  But you're getting away from 
conventional practice there.

(suitable transformers are $120 from McMaster Carr

> ## In the final analysis, cheaper to just use BIG coax.  

Not necessarily if you compare apples to apples (e.g. don't claim "well, 
I have a roll of hardline that I got from the Cable company for free")

And, here's the big deal. FCC regs: the maximum power output *at the 
amplifier output* is 1500 W PEP.  Put the amp at the antenna and you get 
all 1500W to the antenna.  Put the amp 2 dB away, and some of that 1500W 
goes to making the coax warm.

RF watts are almost ALWAYS more expensive than AC watts, if compared 
apples to apples.

However, if you start throwing "free sweat equity" into the mix, it can 

If you need to have the amplifier close to hand (maybe it needs manual 
tuning, maybe it's kind of wonky and needs careful supervision to not 
catch fire, etc.) then that's a reasonable system solution.

An amplifier that requires manual oversight is cheaper than an amplifier 
that can run unattended in a box out in the field:  for instance, rather 
than buy a computer and actuators to tune the output tank on a tube amp, 
you can use the computer between your ears and the actuators at the end 
of your arms.

And, the design and fabrication of a remote amp *is* more complex than 
just digging a ditch and laying 500 feet of coax. It also requires 
different knowledge and skill set. (and may require different regulatory 
oversight... probably don't need a building permit for 500 ft of coax, 
but you'll probably need one for a 500 ft branch circuit.. unless you 
claim it's just a long temporary extension cord)


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