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G8MNY > SWL 28.08.05 18:16l 126 Lines 5019 Bytes #999 (0) @ WW
BID : 59769_GB7CIP
Read: GUEST DK5RAS
Subj: 1W UK MW Station, Aerial system
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0AR<VK6HGR<GB7YFS<GB7CIP
Sent: 050828/1509Z @:GB7CIP.#32.GBR.EU #:59769 [Caterham] $:59769_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : SWL@WW
Hi Readers,
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 ERP that is
allowed is not simple!
This is how we do it...
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.
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´ . ³ \ ³ / ³ .
1.8´ `\ ³ LSB \ ³ / USB ³ /
1.6´ `\ ÀÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÙ /'
1.4´ `. ,-'
1.2´ ` -... _ _ ...-' Measured
1:1´ """ 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,
but this is just about OK of Broadcast AM.
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.
AERIAL EFFICIENCY
_
³
³
1/4 ³
Wave ³ AERIAL TO
³ SAME SCALE
³ ____ 2% efficient
_____³_____ ____\_____ Maximum!
/////////// /////////// + other losses
As the aerial height was only 1/7 of a 1/4 wave, officially I estimated the
maximum aerial gain to be less than 2% (1/7 x 1/7) (-17dBi) as it was only base
loaded.
The Old DECCA transistor LW Tx, on 531KHz was only capable of about 600W PEP
max, above that it was totally non linear (Linear PA OK for CW @ 1KW on LF!).
So about 100 - 150W AM carrier is used. With reasonable estimates for aerial
gain & all the site losses, the off site ERP on paper looks quite close to a
real 1W.
Coverage was to 60 miles with a 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.
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 can't be detected at 300M from the aerial!
(>-100dB ?)
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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