| |
G8MNY > BCAST 09.09.09 00:41l 201 Lines 10102 Bytes #999 (0) @ WW
BID : 36711_GB7CIP
Read: GUEST DG9NBR
Subj: 1W @ 531kHz MW Station System
Path: DB0FHN<DB0FOR<DB0MRW<DB0RGB<DB0PM<OE2XEL<OE6XPE<DB0RES<ON0AR<GB7CIP
Sent: 090908/2146Z @:GB7CIP.#32.GBR.EU #:36711 [Caterham] $:36711_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : BCAST@WW
(8 Bit ASCII Graphics use code page 437 or 850)
Hi Readers, (Updated Oct 07)
For 6 years me & other hams have done six 1 month MW U.K. Restricted
Service Licence broadcasts on 531kHz, (565m) which is more than 3x longer
wavelength than topband, so you may like to know how we do it, especially as
there is a 500kHz experimental UK ham band now.
Only vertical polarisation is used for MW broadcasting, as all domestic Rx are
vertical (vertical car aerials & horizontal Rx ferrites rods), so horizontal
radiating aerials are of no use. Also there is a severe restriction on RSLs,
that limit the aerial height to only 20m at this frequency, so to get a good 1W
MONOPOLE ERP is not at all simple.
This is how we did it...
THE AERIAL
We used a modified inverted L design that was suitable for our site, it has a
45ø sloping vertical underneath the top section. This proves very effective
with next to no horizontal polarization component.
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. e.g. __
\/
TUNING & BANDWIDTH
Loading was done with a large tapped series L & with 3nF across the coax. 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 either side of the coil, fine tunes the aerial system to get this well
centred graph...
A M S I G N A L
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 SWR of 1.5:1, but this is just about OK for Broadcast AM. The Tx AF
system is quite flat 20Hz - 6kHz 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' ú /, \ / \
I ===³P / ' *---* \
S ===³A / / \ / \ú . \ [@] = Loading coil in box.
E ===³C *----[@]*---C---*---R---* C = Centre stake, 1.5M 22mm Cu pipe
R ===³E \ \ /.\ú/ ' / * = >1M 22mm dia Cu Earth pipes
S ===³R. ú\ ' *---* / \/- = Joining wires
\ / \ / R = Rope tie point 1.5m Cu plated steel
*----------* .ú' = Rope to spreader.
<- - - - - 5m - - - - ->
The earth uses 14 short 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
detectable effect to the aerial Z either. I think this was due to the wet
ground conditions 1m underfoot! Putting salt around the copper rods may have
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 at least Small earth
Quarter Wavelength Radius
As the aerial height was only 1/7 of a 1/4 wave tall, 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 ground resistance at the Tx site!
FIELD STRENGTH MEASUREMENTS
As with all aerials, there is near field (cube law) component not square law, &
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
CRT TV line timebase QRM).
N.B. A point source radiator has infinite electric & magnetic fields, but not
at 90ø & hence it has NO ERP.
This near field, has the effect of uncalibrating field strength measurements
for ERP calculations locally (under 10 wavelengths), so local magnetic only
field strength measurement method is not very accurate for estimates of the
real ERP. The methord is unless unless the -6dB/distance doubling can be shown
to be true.
Also affecting the field strength at our site are large variations in the
terrain type over the first few wavelengths (e.g. wet clay valley & 3 nearby
dry chalky hills). These one would expect to affect the radiation resistance of
free space (377ê) near to the ground. Of course in free space at >10 wavelength
away, the field strength measurement would be accurate for estimating ERP.
Both the Aerial & Tx system have been technically inspected by the regulator
Ofcom & were all OK, & the system radiated the correct ERP when measured at a
distant calibrated 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 peak ERP into short aerials!)
So about 50W of AM carrier is needed for the 1W ERP. The broadcast modified Tx
uses a temperature controlled Xtal Osc, to a low power AM exciter stage with
diode clipping of the RF for give the AM, to a 1W tuned class A amp. This then
feeds 6 large class B DC coupled amps pairs with NFB in push pull (36 TO3s in
all), to an iron dust core output 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 (800W CW mode),
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 & has linear phase down to a few Hz.
This is because there is no modulation transformer or LF choke to give LF phase
error. This is very important when used after an AM broadcast limiter that adds
small amounts of near DC components, so not to muck 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
this Tx harmonics are > -60dBc & with the high Q of this aerial system, results
in them being further suppressed to approx -100dBc. They actually can't be
detected over sky noise 400m away from the aerial!
1W ERP RANGE @ 531kHz
Ground wave voverage is about 60-90 miles (100-150km) with a good normal Rx
(only 10 miles 18km @ night due co. ch QRO QRM). With a comms Rx with RF quiet
locations >400 miles (700km). e.g. 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, & 1/9 of the coverage area, a
point possibly missed by the licensing regulator!
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 MW 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
Read previous mail | Read next mail
| |
