Adjust circulators down to the 144 and 432MHz bands

Experience from LA6LCA and LA8OJ is that weaker magnetic field reduces frequency, and stronger field increases the frequency for a circulator on VHF/UHF/SHF.
A lot of circulators have become available from recent dumping of 2m/70cm telecommunication equipment, and although such devices is far away from interest from the
normal dumb FM-operator, I have noticed a lot of interest among my closest friends. Some circulators have as much as 1dB loss, it may be too much for the DX station,
but it isn't so important for a VHF beacon or a packet radio relay which may run with defective antenna for months or a half year without any of the local amateurs noticing
it, and as such save the transmitter.

Normal connection of the circulator

First step to adjust for minimum deflection on RF-mV meter

Second step.

Third step, before reconnecting back to normal and measure the insertion loss

The attenuator is used to improve the impedance of the RF millivoltmeter. The best attenuation on a measurement was 27dB, so it is not problem with a Wavetek model 3001 signal generator and meter reading above 50mV. See LA7MI linear scale RF-mV meter for 0.01-1500MHz on page m2. Some adjustment could be done by adding paper inside to reduce the magnetic field, but you may often find some trimmer capacitors, and in some cases it is neccessary to increase the capacitance to reduce the operational frequency.

LA8OJ visited me today and realigned 3 circulators, two from 148 and 160MHz to 144, and one from 462 to 432MHz

The best circulator - from NEC paging transmitter, easy to align on 144.300MHz

Two stage circulator, somewhat more complicated than neccessary, and more loss than single stage type, 1.5dB loss on 144.2MHz. Need some extra capacitance (10pF) across the trimmer capacitors for optimum operation on 2m band. Possibly useful for beacons. Has built-in dummy-loads and need to be screwed on to cooling block.

NMT450 circulator. Possible to use on 432MHz, but has 1.5-2dB loss. Good enough for beacons.
The 2nd type for NMT450 (Magnetic combiner) has not yet been tested.

Isoductor or Gyrator (Isolator)

See the article by Robert Lentz DL3WR: Zirkulatoren und Isolatoren, abschliessender 2. Teil, UKW Berichte Heft 1 1970 pp 42-46, and Heft 4 1969 pp221-227.

These components are often found in NERA radio link equipment made in the 70's. The most interesting is that they are tunable over a wide frequency range.

Melabs LB-7 isoductor used in NERA 116MHz insert (repeater) modulator (9XM78A), the oscillator transistor to the left and frequency doubler to the right.
The complete 6GHz TX-Oscillator-Group unit is 10XM77A with output on 6296.89MHz. Supply voltage is -24VDC.

Related page:
n21 Wavetek Model 3001
d24 Equipment from pager transmitters

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Last update: 2005.11.14