G8MNY > TECH 04.12.23 08:04z 188 Lines 8053 Bytes #26 (0) @ WW
BID : 61670_GB7CIP
Subj: Piston Absorption Wavemeter
Sent: 231204/0754Z @:GB7CIP.#32.GBR.EURO #:61670 [Caterham Surrey GBR]
To : TECH@WW
By G8MNY (Updated Mar 06)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
A Frequency counter of course will only read one parameter "the frequency of
the strongest signal! This device is used for "seeing RF" Tx RF in your coax
& confirming it is approx. on the right frequency!
Using a scanner Rx to look for abnormal signals you can be lead astray, as it
will have image & harmonic problems. Although a scanner check is very useful,
but it can be..
a) difficult making sure it is not overloaded.
b) Seeing a signal at say +42.8MHz because your Scanner uses say a 21.4MHz IF.
At one time a simple abortion wavemeters like this used to be need for the
Licence to check a Tx was inband & not Tx significant power outside the band or
on its harmonics. e.g. all spurious < 1% power is > 20dB down.
Although absorption wavemeters are not spectrum analysers, they are able to
detecting high levels (e.g. > -30dBc) of harmonics, unwanted mixer products, &
not too "close in" side spurious.
This 1/4 wave line absorption wave meter can detect Tx frequencies from 70MHz
to 1.3GHz in one range. With an accuracy of about 1%. At the higher frequencies
3/4 or 5/4 wavelength resonance modes also give good accuracy.
Powers of 100mW - 50W should be OK mainly limited by the diode PIV.
RF is coupled into a variable length Č wave resonator. At resonance a pickup
loop drives a diode detector to give an indication to a meter. As the length is
varied the exact position of the peak is indicated on a frequency (wavelength)
Drive __ Pickup Feed
Link ( │ Link _Thro
RF Load (o───┐ ( === ┌──┤>├─0_0─────> Pot &
RF Drive (o───┘ ( │ │ │ Meter
1.1M of 25mm Square Aluminium tube.
1.1m of 3 to 5mm Brass rod.
x1 22x22x6mm or thicker Aluminium Blanks (off old heatsink?)
2x Chassis sockets N, SO239, or BNCs
4 Screws eg Steel 4BA/3mm Counter Sunk
8 pop rivets/Self tap screws
2cm of plastic support rod (heat glue stick)
Steel Wire (Coat hanger)
Clip to support Steel wire!
1x Signal diode e.g. 1N4148/914 or better.
10cm of 18-25swg enamelled copper wire
1x 1n feed through cap (bolt in type)
Taps for screws
Pop Rivet Gun (with tube collar!)
Drill & bits for Screw tap & clearance
Wire wool/sand paper
Blow lamp / large heat source
70cm & 2M Tx for calibration
Screws ┌─0─┐ ┌─0─┐
\__│ │ │___│ │ │_______________________________________________________
Blank│ │ └───────┘ Square Tube | | │
│ │ Driving link Spacer| | │
│ │ │ ┌─────────┐Pick up Brass Rod | | │
│ P Clip │Feed C FREQUENCY SCALE
Coat Hanger Wire
SCALE ==o)__P Clip
RF │ // │
Sockets┌───┤ // │ C
│ │= 0 │]---------┐
└───┤ Brass │ POT<--METER
POP Rivets>│ Rod │----------┴---┘
1/ Make an Aluminium blank at least 6mm think to closely fit inside the end of
2/ Fit the blank, drilled 4 tapping sized holes one each side of the tube. Mark
the blanks position & remove.
3/ Tap the blank to take the screws. Drill clearance holes in the tube.
4/ Centrally mark the bank. Drilled hole out & file (needle files) to be a
tight sliding fit to the brass rod.
5/ Drill holes for the 2 RF sockets (pop rivet size/self tap) in the tube near
to the blank as possible, with then as close to each other as possible.
6/ Solder a thick wire to make the Driving link on one of the sockets.
Check that it clears the Brass rod OK. Pop rivet/screw in place in the LOAD
7/ Put the Tx Socket in place Pop rivet cut the link to length & solder up.
8/ Drill a hole for the bolt in feed through capacitor in the tube opposite to
the load socket.
9/ Solder the signal diode with minimum lead to the capacitor. Then solder a
wire to free end of the diode to make the detection loop.
10/Fit the capacitor making sure that all the bits will clear the brass rod.
11/File or saw a slot in the blank to ground the detector wire. Bare the end of
the wire. The wire can be placed under a blank fixing screw if needed.
12/Place the blank & brass rod in place. Push the brass rod so it just
protrudes from the tube open end. Cut the spacer plastic bar to neatly fit
diagonally across the tube. Drill an undersized hole centrally on it. Heat
the brass rod up & force into spacer. This should attach it OK. Otherwise
13/Bend up a coat hanger wire to make the frequency pointer. A few turns
around the brass rod. clean up with wire wool & soldered up will attach it.
14/Put a small cable clip (folded P shaped type) as a guide around the sliding
wire & use a blank fixing screw & washer to hold firmly in place.
15/Attach a sensitive meter (& a pot) to the detector O/P & connect some RF.
If moving the rod you can see a peak, then you can do the calibration.
Two frequencies are needed for this as the electrical end point will not be
the same as the brass rod's physical end (usually outside the tube!).
Infinity 432MHz 144MHz
???? │ │
The distance between 432 & 144MHz marks is exactly 2x the distance from the
Reference scale infinity to 432. Note 432MHz will also produce resonance around
the 144MHz mark as a 3/4 wave resonance. Ignore the longer resonances for now.
End point (144-432 marks mm)
from 432 D432 = ----------------- mm
So place a length light coloured tape under the coat hanger pointer & mark off
all the known frequency lines you can. Accurately measure these & put into a
chart (e.g. a spread sheet) to calculate the length of any frequency.
Distance from D432 * 432
End point for DFx = ---------- mm
Fx MHz Fx
So using just 2M & 70cms markers it is quiet easy to extrapolate the full
Now make a scale chart for 70,75,80,85 90,95,100,110,120,130,140,150,160,170,
200,250,300 400 500 600 700 800 900 1000MHz
1.1, 1.2, 1.3 GHz. Note the high frequencies a very close together & suitable
marking are difficult.
= = = ==
│ ||│||│| │|│||||│ | │||||│| | │ | │ | │ | │||||│ | │|||| │ │ │||||│||||│
ý 1 7 5 4 3 2 2 1 1 1 1 1 1 9 8 8 7 7
G 0 0 0 0 5 0 7 5 3 2 1 0 0 5 0 5 0
H 0 0 0 0 0 0 0 0 0 0 0 0
Then engrave these measurements onto the tube leaving room for a 3/4 & 5/4
wave scale sets if you want to add those in further calibration scales.
I high lighted the ham bands (=) in coloured felt pen. You should find that
they show up as quite wide bands, as 2MHz of 2M is 1cm long. So if you have
been accurate in the scale marking, basic frequency measurement down to about
1% quite possible.
Also look at my TECH buls on "BURNS's MF-UHF Wavemeter TC-101", "AKD's VHF/UHF
Wavemeter WA1", & "RF Field Indicatopr FL-30AH".
Why Don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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