This is one option to homebrew at a reasonable price is a very useable piece of equipment and it has a professional look when finished. If one puts the kit together without making any mistakes then it will work perfectly first time round. A bonus is that the schematic is supplied and you’ll know it functions on the inside. And then afterwards you may opt to apply below mentioned modifications.

The range can be extended above 31 MHz by letting the oscillator works up to 60 MHz. At the high end of those frequencies the output on TP1 is too low to obtain reliable and useful measurements. For up to 50 MHz it is a useable and a worthwhile extension.

If you have to choose between a RF1 analyser or rolling your own from this kit then the latter is the better option.

The antenna analyser was constructed from the VK5JST design of 2004 whenever time permitted. You’ll read about this later on together with the experiments done. If you use google then you’ll find enough of the original kit documentation.

This tool is supplied as a complete kit, inclusive a plastic housing, front, double sided drilled PC board and all components, all screws, spacers, wire, feet, plugs, knobs, battery holders, diagram, construction notes etc. Nothing extra has to be purchased and world wide more than 1000 kits have been supplied. When it arrived no parts were missing and all components had the correct value. The cost of this was $ 130 Australian (approx. 93 Euro at the time of ordering). Put it together without any mistakes and it functions immediately after the simple calibration. My redesigned front (in the picture on the right) looks different from the original model. The "digit" switch was not required and the "power" switch was attached to the top of the PC board.

The kit is supplied with two ordinary banana sockets however my preference is for a coaxial output. I adapted a BNC socket (fig») to this task, which required some innovation to mount D5, D6 and 2 × 100 Ω resistors.

I replaced three 100 pF capacitors with some 50 nF SMD capacitors in order to keep the connections short. However this did not offer a lot of mechanical stability and later these SMD capacitors were replaced by the normal smaller 100 nF type. I placed the on-off switch next to the BNC connector because the BNC connector would offer some protection against accidentally switching the power on.

I found in the junk box a multiple computer connector and used that to connect the LCD to the PCB. It has 13 pins and the required 14^th wire was created from some black hook-up wire and is used for the ground connection.

This connector is useful during experiments and when the LCD gets mounted later on. It is especially useful when the LCD gets mounted on the inside and I used double sided tape for this. This tape (fig») is normally used for mounting mirrors, very durable, very strong and water-resistant. The battery holders were attached to the cabinet using the same tape.

(This tape was selected because many years ago I attached a metal cabinet to the underside of my desk and it is still there, neither are I able to pull it off.)

It looks like the design assumes that the TP2 and TP3 have half the voltage value of TP1. When the circuit diagram is closely studied then this seems to be unlikely. I adjusted TP2 and TP3 to the lower value of 2.14 V @ 2 MHz and this delivered an acceptable overall performance. You may want to experiment with 2.13 – 2.16 V instead of the originally specified 2.25 V.

These results were obtained by using the original calibration values. The frequency reported on the LCD screen (fig») deviates from the generated frequency. I used a "standard" 0.33 µH RFC for the range up to about 54 MHz.

The oscillator drifts slightly. Likely not enough buffers following the oscillator cause this. The measurement results are not constant through all the ranges and this is the result of the different voltage levels offered to the measurement circuit. It was the intention to homebrew a solution but research on the internet revealed that two others already had come up with the answer.

The oscillator requires a separate 5 V dc stabilisation and ALC will ensure a more stable signal being offered to the measurement circuit.

After applying the modifications suggested here the adjustment is no longer sensitive and the last number is either stable or moves up/down by a single amount.

Originally I was considering (fig») a buffer in line with a MAR type amplifier. This has the advantage that less changes to the printed circuit tracks.

In practice adding the MAR amp between Q7 and Q8 did not meet the expectations and similar results were obtained using other discrete amplifiers. The voltage level on TP1 becomes too high and this becomes too much for the PIC processor to function properly.

ALC applied to the oscillator may influence the stability and the regulation of a buffer amplifier is more appropriate. Q9 and Q10 together make up a cascade amplifier where Q10 is bypassed for HF. Q10 operates in an grounded base configuration with low input impedance. One of the possibilities to control the amplification is seen in the diagram.

The track between pin 2 and 3 on IC1 has to be cut (fig») from earth and each other in order to activate IC1d. I used a dentist’s drill for this. Afterwards pin 1 and pin 2 are connected.

The 560 Ω resistor connected to the emitter of Q7 (and base of Q6) is replaced by 330 Ω.

The battery level indicated in the LCD was too low and it was corrected experimentally with a 1 MΩ on the tracked side of the PCB and parallel to the 16 kΩ resistor.

The ALC functions as expected and the signal level on TP1 can be adjusted between approx. 1.58 and 1.7 V.

For the time being the voltage level was adjusted to 1.58 V and the improved results (when compared to the original version) are reported in the table.

It was earlier on noticed that there were likely not enough buffers following the oscillator. One consideration was that the divider might also have an influence on the stability of the oscillator. The 74LS04 have six inverters yet only two were in use. Once a third inverter was activated («fig) the voltage on TP2 increased slightly and this proves that an extra buffer has its benefits.

As a result I have made the experimental change into a permanent one. Pin 4 and 5 are already connected. Only (fig») the track between 4, 5 and going to pin 14 of the 74LS93 has to be cut and afterwards pin 6 is connected to the cut track going to pin 14. (Pin 4 and 5 stay connected with each other.)

The design assumes that the voltage drop across D5 and D6 is equal when the bridge is in balance at 50 Ω. It was noticed that the current through D5 is not equal to the current through D6 (ID5 ≠ ID6) in the Wheatstone bridge. The series 1 k Ω resistor that is in series with D5 causes an imbalance. A 1 kΩ resistor needs to be put in series with D6 to correct this.

The output levels produced by D4, D5 and D6 are measured at pin 14 (TP14), pin 8 (TP8) and pin 7 (TP7) of IC1a.

Before the modification the output level at TP7 was not equal to TP8 (TP7 ≠ TP8) and neither were either of them equal to half the level of TP 14 (TP3 ≠ TP4 ≠ ½ TP14).

The remaining unequal levels of D5 and D6 require a different calibration procedure. The following has proved to be successful: