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G8MNY > BCAST 20.08.06 10:21l 177 Lines 8804 Bytes #999 (0) @ WW
BID : 42916_GB7CIP
Read: GUEST DK5RAS
Subj: 1W @ 531kHz MW Station system
Path: DB0FHN<DB0RGB<DB0MRW<DK0WUE<7M3TJZ<ON0BEL<ON0BEL<GB7CIP
Sent: 060819/1957Z @:GB7CIP.#32.GBR.EU #:42916 [Caterham] $:42916_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : BCAST@WW
Hi Readers, Updated 8/06
Over the last 6 years I & other hams have done five 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 as all domestic Rx are
vertical (Vertical car rods & Horizontal Ferrites), so horizontal radiating
aerials are out. Also there is a severe licence restriction on RSLs, that limit
aerial height to only 20m, so to get a good 1W MONOPOLE ERP is not simple!
This is how we do it...
THE AERIAL
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!
Very_ 3 S p r e a d e r s _,Very
Tall 'Ropes 2m 1m 0.5m Rope Tall
Tree ''===========================================-------------' Tree
\\\\\ 2x 90m of 2.5mm stand Cu. Twisted
\\\\\ Mean height 20m
\\\\\
10x 28m of \\\\\
1.5mm Risers \\\\\
all 20cm spaced \\\\\
& sloping at 45ø \\\\\ (
---->( Tapped (5mm dia tinned Cu wire)
³ ( Large (40 Turns 15cm dia)
8mm Spark Gap ( Loading Coil
³ (
50m UR67 ==============ÄÄ´
³ ÄÁÄ 3nF
³ ÄÂÄ 1kV 15 BONDED
_³__³_____ EARTHING
³³³³³³³³³³ STEAKS
The 2 top wires are tied off in tall trees, really taught at 50kg tension, &
the 10 slopers use thinner wires to carry the current (as we had plenty of that
wire available). We used 3 plastic pipes as spacer bars for the risers & 3
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 AF system is
quite flat 40Hz-7kHz but >-40dB @ 9kHz.
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 uses 18 copper pipes in 2 circles. We found that adding any more did
not alter the aerial Z at all! Also adding a 463ft counter poises had no
detectable effect to the aerial Z either. I think this was due to the wet
ground conditions 1m underfoot! Watering & salting the copper rods can also
help keep the earthing losses low in dry summers.
AERIAL EFFICIENCY & ERP
_
³
³
1/4 ³ IDEAL
Wave ³ REFERENCE AERIAL USED TO
³ AERIAL THE SAME SCALE
³ ____ 2% efficient
_______________³_______________ ___________\___________ Maximum!
/////////////////////////////// /// + other losses
Good Cu Ground Mat Over Several 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 (5km) & not just the local
resistance!
As with all aerials, there is near field (cube law) component & this should be
much stronger with a tiny aerial, than with a full size aerial. This is due to
the "transformer action" (like local TV timebases).
N.B. point source radiator has infinite electric & magnetic fields but not at
90ø & has NO ERP.
This has the effect of uncalibrated any local field strength measurements, so
that method is not a very accurate method of estimating the real ERP. Also
affecting the field strength method at our site are large variations in the
terrain over a the first few wavelengths (eg. wet clay valleys & nearby dry
chalky hills). Of course in free space at >10 wavelength field strength
measurement are an accurate at estimating ERP. Both the Tx & Aerial system have
been technically inspected by the regulator Ofcom & were all OK.
THE TX
This is a large old DECCA transistor LW MCW Beacon Tx, pushed up to work on the
edge of its frequency range at 531kHz, & it is 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 needed for the 1W ERP. In it's 1st life these Tx can
produce 10W Monopole ERP (CW) with a tall 140m tower @ 250-300kHz, where
modulation linearity was not important for a CW beacon.
The broadcast modified design uses temperature controlled Xtal Osc, to a low
power AM exciter stage, to a 1W tuned class A amp, then to 6 large class B amps
pairs with NFB in push pull (36 T03s) to an iron dust core O/P transformer.
Across this is a large permeability tuned L (6 ferrite rods) that resonates 24
caps across each PA transistor.
ÚÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄ¿
³STABLEÃÄ´DIVIDERSÃÄ´RF CLIPPINGÃÄ´TUNNEDÃÄÄÄ´ PA1-3 ÃÄÄÄÄÂÄÄÄÂÄÄÄÄÄ¿ ÚÄ(o
³ OSC ³ ÀÄÄÄÄÄÄÄÄÙ ³ MODULATOR ³ ³ AMP ÃÄ¿ ÀÄÄÄÄÄÄÄÙ === )|/\ )::( ³
ÀÄÄÄÄÄÄÙ ÀÄÄÂÄÄÄÄÂÄÄÄÙ ÀÄÄÄÄÄÄÙ ³ ÚÄÄ´ )| HT-)::( ³
³ ³ ³ ÚÄÄÄÄÄÄÄ¿ Á === /)| )::(__³
SET CARRIER POT>ÄÙ ³ ÀÄ´ PA4-6 ÃÄÄÄÄÁÄÄÄÁÄÄÄÄÄÙ Á
SET MOD POT ÀÄÄÄÄÄÄÄÙ
STUDIO ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ ³ ÚÄÄÄÄÄÄÄÄ¿
AF ÄÄÄ´FILTER+LIMITERÃÄÄÄÄÙ MAINS Ä´ SMPSU1 ÃÄ>+115v -6 +6 +12
FEED ÀÄÂÄÄÄÄÄÄÄÄÄÄÄÄÙ ÃÄÄÄÄÄÄÄÄ´] ÚÁÄÄÁÄÄÄÁ¿
GAIN Broadcast MAINS Ä´ SMPSU2 ÃÄ>0-55V >ÄÄ´INVERTERÃÄ+20
Limiter ÀÄÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÄÙ
The original HOT linear PSUs were 55V + 60V @ 4A, 12A peak, are superseded with
2nd hand SMPSU ones that run cold to reduce unwanted shack heating!
Although a very inefficient Tx design (10% at 50W on 531kHz) compared to high
level AM mod, it does do very good bass. This is because there is no modulation
transformer or LF choke to give LF phase error after the AM broadcast limiter
to much up the modulation waveform. Only efficient PWM system are as good.
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 (100km) with a good normal Rx (only 15 miles @ night
due co. ch). With a comms Rx with quiet locations >400 miles (700km). eg. we
have Rx Dx reports from Finland, N Italy, N.Scotland, Channel Isle etc.
This seems extreme DX for a QRP MW station, but it would only be 1/3 of this
range @ 1593kHz, and 1/9 of the coverage area! Aren't laws of physics
wonderful.
However QRM from any SMPSU, TV timebase, Broadband cabling, & Fluorescent
lamps can easily wipe out a weak signal on this frequency.
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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