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G8MNY > BCAST 28.03.07 02:39l 187 Lines 9367 Bytes #999 (0) @ WW
BID : 8382_GB7CIP
Read: DK5RAS GUEST OE7HNT
Subj: 1W @ 531kHz MW Station system
Path: DB0FHN<DB0MRW<OK0PKL<OK0PCC<OM0PBC<OK0PPL<DB0RES<ON0AR<GB7CIP
Sent: 070327/2356Z @:GB7CIP.#32.GBR.EU #:8382 [Caterham] $:8382_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : BCAST@WW
Hi Readers, (Updated Aug 06)
Over the last 6 years I & other hams have done six 1 month MW U.K.
Restricted Service Licence broadcasts on 531kHz. 531kHz (565m) is more than 3x
longer wavelength than topband, so you may like to know how we do it,
especially as a 500kHz ham band only looks a few years away.
Only vertical polarisation is used for MW broadcasting as all domestic Rx are
vertical (vertical car rods & horizontal Rx ferrites rods), so horizontal
radiating aerials are out. Also there is a severe restriction on RSLs, that
limit the aerial height to only 20m, so to get a good 1W MONOPOLE ERP is not at
all 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 7 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 (20 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 40kg tension, &
the 10 slopers use thinner 7 stranded 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 spacer triangles. eg. __
\/
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, but dressing the earth & aerial
wire fine tunes the aerial system 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 20Hz - 7kHz ñ1dB but > -40dB @ 9kHz. Treble around 5-7kHz will
produce phase modulated sidebands into this aerial reducing the effective AM
modulation depth!
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 earth uses 14 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 like this, than with a full size ¬ or « wave
aerial. This is due to the "transformer action" (like local TV line timebase).
N.B. A point source radiator has infinite electric & magnetic fields, but not
at 90ø & has NO ERP.
This near field, has the effect of uncalibrating field strength measurements
for ERP calculations locally, so that method is not accurate estimates of the
real ERP. Also affecting the field strength at our site are large variations in
the terrain type over the first few wavelengths (eg. wet clay valley & 3 nearby
dry chalky hills). Of course in free space at >10 wavelength away the field
strength measurement is accurate for estimating ERP.
Both the Aerial & Tx system have been technically inspected by the regulator
Ofcom & were all OK, & radiated the correct ERP when measured at a distant
calibraited site.
THE TX
This is a large old DECCA transistor LW MCW aircraft navigation 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. (The PA is
OK for non linear CW @ 800W on LF to produce 10W ERP!)
So about 50W of AM carrier is needed for the 1W ERP. The broadcast modified
design uses a temperature controlled Xtal Osc, to a low power AM exciter stage
with diode clipping the RF for give the AM, to a 1W tuned class A amp, then to
6 large class B DC coupled amps pairs with NFB in push pull (36 TO3s in all),
to an iron dust core O/P transformer. Across this is a large permeability tuned
L (6 movable ferrite rods) that resonate the 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 2 HOT linear PSUs were 55V + 65V @ 4A, 12A peak, are superseded
now by two 2nd hand SMPSU ones that run cold, & 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 & linear phase down to a few Hz. 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 further suppressed. They actually can't be detected over sky noise 400m
away 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 QRM). With a comms Rx with RF quiet locations >400 miles (700km).
eg. we have Rx Dx reports from Finland, 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 a point possibly missed by the
regultor! Aren't laws of physics wonderful.
However QRM from any SMPSU, PC Screens, TV timebase, Broadband phone cabling, &
Fluorescent lamps, can easily wipe out a weak signal on this frequency.
Also see my bul on "AM Broadcast Radio Principles"
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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