Hi J, Let us go back to the laws of physics which basically say you cant get
something for nothing !! If you are feeding in 150 watts (that is the max
assuming 100% efficiency) and getting 2.8 amps of antenna current at
resonance, the antenna system resistance is 19 ohms (or less). I am afraid I
said before that fitting a transformer was a snare and delusion. I think
Alex has the point that part of its reactance is being used to tune the
antenna so it is not really 1:1.
Now ERP, you are running 2.8 amps of antenna current which is fine. Your
ERP will depend on the radiation resistance of the the antenna. You can get
a rough estimate from the height of the vertical. Let us work backwards 2.8
squared is 7.84 so that if you radiation resistance is more than 1/7.84 (
=0.128 ohm) you are over 1 Watt ERP even with only 150 watts input. My guess
with 100 foot of vertical is you may have as much as 0.300ohm Rrad , but
will I need to find the formula for a top-loaded vertical again. This means
you may well have 2w ERP at present (this all assumes the RF ammeter is
correct). A temporary connection of the ammeter above the loading coil might
make a good confidence test, as there is no possiblilty of measuring
non-radiating currents at that point.
I caution that the figures you have been absorbing from the books are from
mainly from UK style antennas which are much shorter than yours. Out normal
antenna heights over here for domestic sites are 30 to 50 feet. A 100 foot
antenna has 4 times the Rrad of a 50 footer. and about 8 times the Rrad of a
30 footer. I recollect figures for a 30 foot antenna working out in the 60
to 80 milliohm range. Hence the need for 1kW input to achieve 1W ERP.
Cheers de Alan G3NYK
----- Original Message -----
From: J. Allen <[email protected]>
To: <[email protected]>; LowFER at LW Cafe <[email protected]>
Cc: Scott Tilley <[email protected]>; Steve McDonald <[email protected]>
Sent: 05 October 2006 03:55
Subject: LF: Math HELP!
> Calculating Wiz Folks,
>
> Steve lent me an RF ammeter. It is in the line from the loading coil to
the
> ground system and sees only the current from the antenna. The coax going
> out to the antenna has a different return from its side of the matching
> transformer. The transformer surprised me by needing to be a 1:1
> transformer for 50 Ohms. The system is matched as shown on the ScopeMatch
> scope screen both in level and phase. This antenna plays and receives
very
> well.
>
> A while back I put the description of the antenna complete with drawing on
> the site and was told that the current for 1W ierp should be about 2.85
> amps.
>
> The transmitter power is about 150 Watts output through a 1:1 transformer
> and the current on the antenna side is 2.8 amps now. That corresponds to
a
> power level of ~400 Watts at 50 Ohms.
>
> What in heck is going on? Where should I look for mistakes?
>
> The antenna is a supported by a109 ft tower with a small HF tribander at
> the top. The tower is used as a support but may be coupled in somehow.
The
> vertical portion of the LF antenna is mounted about one meter from the
tower
> and runs from the top of a single layer solenoid loading coil inductor at
> ground level, up the tower on insulators to 110 feet and connects to two
> 390 foot horizontal wires which are supported at their far ends one at 63
> feet and the other at 70 feet. There is about 10 ft of sag in the wires
> between the tower and their respective supports. There are two 3-meter
> ground rods spaced about 3 meters apart and a set of about 40 random
length
> radials from the base of the tower grounded to the radials at its base,
out
> to lengths from say 30 ft to as much as several hundred feet. The coax
feed
> goes through a close coupled 1:1 air wound solenoid matching transformer
to
> ground and the other winding of the transformer goes from ground at the
> radial connection.
>
> Is that number of 2.85 amps correct? If it is, then 150 Watts is my
legal
> limit! This is not what folks explained to me earlier. Please help me
with
> this math.
>
> If I measure 2.8 amps now, does this mean the I am at ~1W ierp? I do
not
> know how to measure IERP at any frequecy, especially LF, and was hoping to
> read the ammeter and go by it.
>
> Thanks,
>
> J.
> VY1JA
>
>
>
>
>
|
