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G8MNY  > BCAST    15.05.22 08:21z 219 Lines 11064 Bytes #28 (0) @ WW
BID : 21722_GB7CIP
Subj: 1W @ 531kHz MW Station System
Path: SP7YDD<SR1BSZ<OK0NBR<OK2PEN<N3HYM<VE3CGR<KA1VSC<GB7CIP
Sent: 220515/0813Z @:GB7CIP.#32.GBR.EURO #:21722 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : BCAST@WW

(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Hi Readers,                                 (Updated Sep 11)
       A few years ago I & several other hams did the engineering on 6 one
month MW U.K. Restricted Service Licence broadcasts on 531kHz. The 565m wave
length was more than 3x that of topband, so you may like to know more,
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 ferrite 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
45° sloping vertical underneath the top section. This proves very effective
with next to no horizontal polarisation component radiated.

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 \\\\\
     7 strand Risers \\\\\
      all 20cm spaced  \\\\\
       & sloping at 45°  \\\\\ (
                          ---->( Tapped  (5mm dia tinned Cu wire)
                            │  ( Large   (20 Turns 15cm dia)
               8mm Spark Gap:  ( Loading Coil
                            │  (
      50m UR67 ==============──┤
                            │ ─┴─ 3nF
                            │ ─┬─ 1kV  13 BONDED
                           ┌┼┬┬┼┬┬┬┬┐  EARTHING
                           ││││││││││  STEAKS

The 2 top wires were tied off in tall trees, really taught at 40kg tension. The
10 slopers used thinner 7 stranded wires to carry the current (as we had plenty
of that wire available). We used 3 plastic 5cm pipes as spacer bars for the
risers & 3 weight stabilised top spacer triangles. e.g. __ to stop the 2 pair
twisting.                                               \/

TUNING & BANDWIDTH
Loading was done with a large tapped series L & with a mica 3nF across Tx coax.
The tapping point (copper strip with wires attached) is moved up, down & around
the coil around until the return loss of > 35dB (SWR 1.04:1) was found. Then
the tapping point was soldered onto the coil & retested. Normally there was
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
              │ \      Carrier      / │
              │   \       │       /   │
VSWR          │     \     │     /     │
2.0:1┤ .      │       \   │   /       │    .
1.8:1┤  `\    │   LSB   \ │ /   USB   │   /
1.6:1┤    `\_ └───────────┴───────────┘_/'
1.4:1┤       `._                    ,-'
1.2:1┤          ` -... _     _ ...-'  Measured
1.0:1┤                   """            SWR
     └┬───┬───┬───┬───┬───┬───┬───┬───┬───┬─
     521 523 525 527 529 531 533 535 537 539 kHz

Tuning was very critical & only a narrow aerial bandwidth of ˝6kHz was possible
up to an SWR of 1.5:1, but this was just about OK for Broadcast AM.

The Transmitters's AF system we used was very flat 10Hz - 6kHz ˝1dB, but >-40dB
@ 9kHz to meet the broadcast spec. N.B. Treble around 5-7kHz produces phase
modulated sidebands on this aerial, reducing the effective AM modulation depth!

EARTHING SYSTEM
               *---------*
R ===│S'  ˙  /,  \      /  \
I ===│P    /    ' *---*      \
S ===│A  /       / \ / \˙ .    \      [@] = Loading coil in box.
E ===│C *----[@]*---C---*---R---*       C = Centre stake, 1.5m 22mm Cu pipe
R ===│E  \       \ /.\˙/ '     /        * = >1M 22mm dia Cu Earth pipes
S ===│R.  ˙\   '  *---*      /        \/- = Joining wires
             \  /       \  /            R = Rope tie point 1.5m Cu plated steel
               *----------*           .˙' = Rope to spreader.
        <- - - - - 5m - - - - ->

This uses 12 short copper pipes in 2 circles. We found that adding any more did
not alter the aerial Z at all! Also adding long counter poises had no
detectable effect to the aerial Z either. I think this was due to the wet
ground conditions 1m underfoot! Putting a few kg of salt around the copper rods
(not on the connections!) may have also help keep the earthing losses low in
dryer summers.

AERIAL EFFICIENCY & ERP
               _
               │
               │
               │
           1/4 │  IDEAL
          Wave │  REFERENCE              AERIAL USED TO
               │  AERIAL                 THE SAME SCALE
               │                               ___             2% efficient
_______________│_______________     ___________\___________      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 was very close the radiation resistance method of calculation that gave
2.9%. But really the true aerial efficiency is all about ground surface
resistance at that frequency over something like 10 wavelengths radius (5km) &
not just at the Tx site!

FIELD STRENGTH MEASUREMENTS
As with all aerials, there was near field (cube law) component not square law,
& this will be much stronger, with a tiny aerial like this, than it would be
with a full size quarter or half wave aerial. This was due to the "transformer
action" (like local CRT TV line timebase QRM etc.)

N.B. A point source radiator has infinite electric & magnetic fields, but not
at 90° to each other, & 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 method was useless, unless a -6dB/distance doubling, can be shown
to be true.

Also affecting the field strength at our site were large variations in the
terrain type over the first few wavelengths (e.g. wet clay valley & 3 nearby
dry chalky hills). These would expect to affect the radiation resistance of
free space (377ŕ) 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 near the correct ERP when measured
at a distant calibrated site.

THE TX
This was a large old DECCA Beacon 80002A LW MCW aircraft navigation beacon Tx,
pushed up to work on the edge of it's frequency range at 531kHz. It was only
capable of about 400W PEP max at that frequency, above that it was totally non
linear for AM. (The PA was OK for non linear CW up to 800W PEP on LF to produce
10W ERP into 100ft tower aerials!)

About 50W of AM carrier was needed for the 1W ERP station. 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 envelope, this feeds a 1W
tuned class A stage, which then feeds split phase into 6 large class B DC
coupled amps pairs with NFB in push pull (36 TO3s in all), to a large iron dust
core output transformer. Across this is a large permeability tuned L (6 movable
ferrite rods) that resonate with the 24 caps across each PA transistor.

┌──────┐ ┌────────┐ ┌───────────┐ ┌──────┐   ┌───────┐
│STABLE├─┤DIVIDERS├─┤RF CLIPPING├─┤TUNNED├───┤ PA1-3 ├────┬───┬─────┐    ┌─(o
│ OSC  │ └────────┘ │ MODULATOR │ │ AMP  ├─┐ └───────┘   ===  )|/\   )::(  │
└──────┘            └──┬────┬───┘ └──────┘ │           ┌──┤   )|  HT-)::(  │
                       │    │              │ ┌───────┐ ┴ === /)|     )::(__│
      SET CARRIER POT>─┘    │              └─┤ PA4-6 ├────┴───┴─────┘      ┴
                       SET MOD POT           └───────┘
STUDIO  ┌──────────────┐    │            ┌────────┐
AF   ───┤FILTER+LIMITER├────┘     MAINS ─┤ SMPSU1 ├─>+120V   -6V +6V +12V
FEED    └─┬────────────┘                 ├────────┤]          ┌┴──┴───┴┐
        GAIN  Broadcast           MAINS ─┤ SMPSU2 ├─>0-55V >──┤INVERTER├─+20V
               Limiter                   └────────┘           └────────┘

The original 2 HOT linear PSUs were 55V + 65V @ 4A, 12A peak (800W CW mode),
were superseded by two 2nd hand SMPSU ones that run cold, & reduce unwanted
shack heating!

EFFICIENCY & MODULATION 
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 was very important when used after an AM broadcast limiter that
adds small amounts of near DC LF components, so this design does not change the
wanted modulation waveform at all. Only efficient PWM systems are as good.

As the low level AM modulation stage can't produce 100% modulaton (always some
carrier left), the class B PAs are left slightly under biased in this design.
This results in good deep carrier cuts on over modulation. But when set up
properly with the broadcast limiter, near 100% modulation peaks are maintained
at near clipping all the time, keeping the AM channel full of deep modulation &
adjacent channels clear of over modulation splatter.

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
At this frequency it was found the ground wave coverage was about 60-90 miles
(100-150km) with a good normal Rx, but only 8 miles 14km @ night due to QRO QRM
from 3 100kW co-channel stations.

With a comms Rx with RF quiet loations >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 these reports are not really at a good listenable
strength.

The range would only be 1/3 of this at the other end of MW range on 1593kHz, &
give only 1/9 of the coverage area! A point possibly missed by the licensing
regulator! Aren't the 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


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