William_H
Well-known member
This instrument is used to test the performance of an antenna. Unfortunately they appear to be hard to get of recent times. They were o0nce common when CB radio was in its heyday. They may now be found possibly at Amateur radio suppliers. They were quite simple (so cheap) but I havn't seen them for sale for a long time.
I recalled an electronics magazine article describing how to make one so thought I would try it. I can hardly say it is perfected but some may wish to experiment to build their own as follows.
The tester fits between the radio and the antenna cable. So you need about 1 metre of coax with a male connector on one end and a male connector plus F to F barrel or a female connector on the other end.
Before you fit the last connecor you need to strip the outer plastic coating off the cable for about 20 cms. The braid is bunched up so that 2 pieces of insulated wire can be fed under the braid paralell to the centre conductorwith the ends emerging through the braid about 20 cms apart. The braid is pulled tight and a piece of heat shrink tubing put over the braid with holes for the 2 wires to emerge. (and a connection to the braid at each end) These wires need to be fairly thin but insulated. I used some stranded wire from 8 core computer interconnect LAN cable.
One end of one wire is terminated with a 50 ohm resistor to the braid while the other end of the other wire similarly is terminated to a 50 oh resistor. A germanium diode connects to the other ends of each wire with a small capacitor 150 pf to ground form the cathode end of the diode.(cathode has the little band on that end) I used a tiny terminal strip for the wires.
Germanium diodes (OA 91) have a lower forward volt drop. Silicon diodes may suit. So each wire has a recifier diode on one end and a terminating resistor at the other end. The diodes being on opposite ends for each wire.
The diode cathodes are connected to wires which lead away with an earth wire to the meter.
I tried using my trusty Digital Voltmeter for measuring the produced voltage with a transmitter connected and transmittting. but it seemed to go berserk with the RF energy so it seems you need an analogue moving coil meter. You can often recover them from an old tape cassette recorder deck. You need resitors of about 1500 ohms from each diode to the meter.
So in operation one wire measures the power going out to the antenna while the other wire measures the power returning from the antenna. Ideally the return should be zero. In practice a meter reading of less than 20 % is good.
The meter is set up with a switch to measure the DC voltage at either diode cathode. A potentiometer can be fitted in series with the meter to enable the meter to be adjusted to full scale for the forward direcion so that the smaller reading of the other diode is reflected power.
It is usefull to build a dummy load for the testing. This is a 50 ohm resistor across the short cable out of the connector which is connected in place of the antenna. This resitor has to be capable of absorbing your 25 watts of trnasmitter power. Or easier and safer to do all testing on transmitting 1Watt level. (on an unused channel) That way the resistor can be a 50 ohm 1watt resistor or 4 x50ohm half watt in series and paralell.
The device is OK if used on 25 watt level but gives obviously much more reading than 1watt. It can be used on HF but you will have to use the tune mode on a SSB to get a steady radiated power. You would need a longer pickup length for use on 2mhz channels. It can work very well on the Australian 27mhz marine band.
With a dummy load connected to the resistor you should get quite a small reflected power. If the resistor is disconnected you should get equal forward and reverse power and likewise if it is shorted out.
The whole device can be reversed so that the end that went to the transmitter is swapped for the other end. The diode giving max reading should then also swap over.
There may be some non symetry about the readings because of non symetry in the pick up wires.
The device should serve to test if an antenna and feed cable is OK or faulty. The idea being to build one and test it so you know what to expect on a good antenna.
One of the problems of this kind of testing is that the cable which goes up to your mast head will waste power. (especially if it is long and the thin coax) This loss also dissipates the reflected power so if you had a very long cable it would measure good even without an antenna fitted. If you can insert it close to the antenna then you get more accurate indication but that is not always easy.
The tester itself will not affect the signal so can be left in permanently but of course it is a good point for corrosion to enter the cable. Or other faults to be introduced.
The meter itself can be fitted in a plastic box on conveneient length wires.
If anyone has the urge to try this DIY out let me know. My appologies for not giving more details. Obviously it is only for the electronics inclined people but on the other hand it is cheap enough to experiment. olewill sorry about the ravings but someone may be interested.
I recalled an electronics magazine article describing how to make one so thought I would try it. I can hardly say it is perfected but some may wish to experiment to build their own as follows.
The tester fits between the radio and the antenna cable. So you need about 1 metre of coax with a male connector on one end and a male connector plus F to F barrel or a female connector on the other end.
Before you fit the last connecor you need to strip the outer plastic coating off the cable for about 20 cms. The braid is bunched up so that 2 pieces of insulated wire can be fed under the braid paralell to the centre conductorwith the ends emerging through the braid about 20 cms apart. The braid is pulled tight and a piece of heat shrink tubing put over the braid with holes for the 2 wires to emerge. (and a connection to the braid at each end) These wires need to be fairly thin but insulated. I used some stranded wire from 8 core computer interconnect LAN cable.
One end of one wire is terminated with a 50 ohm resistor to the braid while the other end of the other wire similarly is terminated to a 50 oh resistor. A germanium diode connects to the other ends of each wire with a small capacitor 150 pf to ground form the cathode end of the diode.(cathode has the little band on that end) I used a tiny terminal strip for the wires.
Germanium diodes (OA 91) have a lower forward volt drop. Silicon diodes may suit. So each wire has a recifier diode on one end and a terminating resistor at the other end. The diodes being on opposite ends for each wire.
The diode cathodes are connected to wires which lead away with an earth wire to the meter.
I tried using my trusty Digital Voltmeter for measuring the produced voltage with a transmitter connected and transmittting. but it seemed to go berserk with the RF energy so it seems you need an analogue moving coil meter. You can often recover them from an old tape cassette recorder deck. You need resitors of about 1500 ohms from each diode to the meter.
So in operation one wire measures the power going out to the antenna while the other wire measures the power returning from the antenna. Ideally the return should be zero. In practice a meter reading of less than 20 % is good.
The meter is set up with a switch to measure the DC voltage at either diode cathode. A potentiometer can be fitted in series with the meter to enable the meter to be adjusted to full scale for the forward direcion so that the smaller reading of the other diode is reflected power.
It is usefull to build a dummy load for the testing. This is a 50 ohm resistor across the short cable out of the connector which is connected in place of the antenna. This resitor has to be capable of absorbing your 25 watts of trnasmitter power. Or easier and safer to do all testing on transmitting 1Watt level. (on an unused channel) That way the resistor can be a 50 ohm 1watt resistor or 4 x50ohm half watt in series and paralell.
The device is OK if used on 25 watt level but gives obviously much more reading than 1watt. It can be used on HF but you will have to use the tune mode on a SSB to get a steady radiated power. You would need a longer pickup length for use on 2mhz channels. It can work very well on the Australian 27mhz marine band.
With a dummy load connected to the resistor you should get quite a small reflected power. If the resistor is disconnected you should get equal forward and reverse power and likewise if it is shorted out.
The whole device can be reversed so that the end that went to the transmitter is swapped for the other end. The diode giving max reading should then also swap over.
There may be some non symetry about the readings because of non symetry in the pick up wires.
The device should serve to test if an antenna and feed cable is OK or faulty. The idea being to build one and test it so you know what to expect on a good antenna.
One of the problems of this kind of testing is that the cable which goes up to your mast head will waste power. (especially if it is long and the thin coax) This loss also dissipates the reflected power so if you had a very long cable it would measure good even without an antenna fitted. If you can insert it close to the antenna then you get more accurate indication but that is not always easy.
The tester itself will not affect the signal so can be left in permanently but of course it is a good point for corrosion to enter the cable. Or other faults to be introduced.
The meter itself can be fitted in a plastic box on conveneient length wires.
If anyone has the urge to try this DIY out let me know. My appologies for not giving more details. Obviously it is only for the electronics inclined people but on the other hand it is cheap enough to experiment. olewill sorry about the ravings but someone may be interested.
