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G8MNY > TECH 10.09.05 13:49l 145 Lines 6370 Bytes #999 (0) @ WW
BID : 61602_GB7CIP
Read: GUEST DL1LCA DK5SG OE7FMI
Subj: 1W UK MW Station, Aerial system
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0AR<GB7CIP
Sent: 050910/1053Z @:GB7CIP.#32.GBR.EU #:61602 [Caterham] $:61602_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
Hi Readers, Updated 9/05
I have little ham experience of MF, but I over the last 4 years I have done
four 1 month MW U.K. Restricted Service Licence broadcast stations on 531KHz
(SUSY RADIO see bul). 531KHz (565m) is more than 3x longer wavelength than
topband!
Only vertical polarisation is used for MW broadcasting, so horizontal radiating
aerials are of no use. Also there is a severe licence restriction on RSLs that
restricts the aerial height to only 20m, so to get a good 1W MONOPOLE ERP that
is allowed is not simple!
This is how we do it...
THE AERIAL
Very_ Spreaders _,Very
Tall 'Ropes 2M 1M 0.5m Rope Tall
Tree ''===========================================-------------' Tree
\\\\\ 90m of 2.5mm Cu. Twisted
\\\\\ Mean height 20m
\\\\\
\\\\\
10x 1.5mm Risers \\\\\
all 20cm spaced \\\\\
& sloping at 45ø \\\\\ (
---->( Tapped (6mm dia wire)
³ ( Large (40 Turns)
8mm Spark Gap ( Loading Coil (15cm dia)
³ (
50m UR67 ========´
³ ÄÁÄ 3nF
³ ÄÂÄ 15 BONDED
_³__³_____ EARTHING
³³³³³³³³³³ STEAKS
We use a modified inverted L, with a 45ø sloping vertical underneath the top
section. This proves very effective with no horizontal polarization & suitable
for our site!
The 2 top wires were about 90m long, tied off in tall trees (2.5mm^2 really
taught (50Kg), & the sloper was 10 spaced thinner wires (1.5mm^2) to carry the
current (as we had plenty of that available). We used 3 plastic pipes as spacer
bars for the risers & weight stabilised top spaces.
TUNING & BANDWIDTH
Loading was by a large tapped series L with 3nF across the coax as well. The
tapping point (copper strip with wires attached) is moved up & down & around
the coil around until the return loss is > 35dB (SWR 1.04:1) is found. Then the
tapping point is soldered on to the coil & retested. Normally there is some
frequency offset of a few KHz to this process, so it is repeated knowing the
offset to get this well centred graph.
³ \ Carrier / ³
³ \ ³ / ³
VSWR ³ \ ³ / ³
2.0:1´ . ³ \ ³ / ³ .
1.8:1´ `\ ³ LSB \ ³ / USB ³ /
1.6:1´ `\ ÀÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÙ /'
1.4:1´ `. ,-'
1.2:1´ ` -... _ _ ...-' Measured
1.0:1´ """ SWR
ÀÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄ
521 523 525 527 529 531 533 535 537 539 KHz
Tuning is very critical & only a narrow aerial bandwidth of ñ6KHz is possible,
up to 1.5:1, but this is just about OK of Broadcast AM. The Tx system is >-40dB
@ ñ9KHz.
EARTHING SYSTEM
*---------*
R ===³S' . / \ / \
I ===³P / ' . *---* \
S ===³A / / \'/ \ . \ [@] = Loading coil in box.
E ===³C *----[@]*---C---*---R---* C = Centre stake, 1.5M 22mm Cu pipe
R ===³E \ \ /.\ / ' / * = >1M 22mm dia Cu Earth pipes
S ===³R . \ ' *---* / \/- = Joining wires
\ / \ / R = Rope tie point 1.5M Cu plated steel
*----------* .' = Rope to spreader.
<- - - - - 5m - - - - ->
The earths use copper pipes in 2 circles. We found that adding any more did not
alter the aerial Z at all! Also adding 463ft counter poises had no effect. I
think this was due to the wet ground conditions 1m underfoot! Watering &
salting the copper rods can also help keep the earth mat losses low in dry
summers.
AERIAL EFFICIENCY & ERP
_
³
³
1/4 ³ IDEAL
Wave ³ REFERENCE AERIAL USED TO
³ AERIAL SAME SCALE
³ ____ 2% efficient
______________³______________ ___________\___________ Maximum!
///////////////////////////// /// + other losses
Good Ground Mat Over Seveal Small earth
Wavelengths
As the aerial height was only 1/7 of a 1/4 wave, the maximum aerial gain is in
the order of 2% (1/7 x 1/7) (-17dBi) as it was only base loaded. This figure is
very close the radiation resistance method of calculation that gives 2.9%. But
really the true aerial efficiency is all about ground surface resistance at the
frequency, over something like 10 wavelengths radius & not just the local
resistance!
As with all aerials there is near field (cube law) component & this should be
much stronger than with a full size aerial due to "transformer action" (like TV
timebases). This uncalibrates local field strength measurements, so it is not a
very accurate method of estimation. Also affecting the field strength method
are variations in the terrain, (eg wet clay valleys & nearby dry chalky hills).
Of course in free space it is easy & accurate.
THE TX
The Old DECCA transistor LW Tx, on 531KHz was only capable of about 400W PEP
max, above that it was totally non linear (Linear PA OK for CW @ 800W on LF!).
So about 50W AM carrier is used for 1W ERP. In it's 1st life these Tx can
produce 10W Monopole ERP (CW) with a tall 140M tower @ 250-300KHz, where
linearity was not important as a CW beacon.
HARMONICS
With this frequency the 2nd & 3rd harmonics are actually in band (MW)! With the
Tx harmonics are > -60dB & the high Q of this aerial system results in them
being greatly suppressed. They actually can't be detected over sky noise at
400M from the aerial! (>-100dB ?)
1w ERP RANGE @ 531KHz
Coverage is about 60 miles with a good normal Rx (only 15 miles @ night due co.
ch). With a comms Rx with quiet locations >400 miles. eg. we have Rx Dx reports
from Finland, N Italy, N.Scotland, Channel Isle etc.
This seems extream for a QRP MW station, but it would only be 1/3 of this range
@ 1593KHz, and 1/9 the coverage area!
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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