Jim Thomson wrote:
> Date: Tue, 13 Apr 2010 21:53:49 -0700
> From: jimlux <firstname.lastname@example.org>
> 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?
> ## 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
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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