|VK5JST HOMEPAGE- PART 12|
|VK5JST VHF AERIAL ANALYSER|
This project was published in the December 2015 issue of the Australian magazine "Amateur Radio", and you can obtain an excellent and complete kit of parts (including the printed circuit boards and pre-programmed Picaxe processor) from the Adelaide Hills Amateur Radio Society (AHARS) . This kit is brilliant value, and actually sells for less than the price you would pay for the individual parts. All profits go to the club too, and this link takes you there........ AHARS Kit.
The kit has been designed using parts which are very readily available (see the circuit and parts list) and so there is little point in supplying a kit of parts to amateurs who have been
around a while - most will be in your junk box. However, the printed circuit boards (double sided, plated through hole and 60/40 Pb/Sn solder reflow coated) are another matter,
and are available here at near cost to hobbyists only.
BEFORE YOU ORDER BY PAYPAL, check that the boards are in stock by emailing me (Jim) at the following address, and then, and only then, click on the Paypal button below. I really don't want to have to issue Paypal refunds!!
PLEASE NOTE THAT ORDINARY AIRMAIL IS USED FOR SHIPMENT. THIS MEANS THAT ALL FORWARDING OCCURS AT YOUR RISK UNLESS YOU SPECIFICALLY REQUEST (AND PAY EXTRA FOR) FORWARDING BY REGISTERED MAIL.
VHF AERIAL ANALYSER PRINTED CIRCUIT BOARDS.........$A25 plus post and packing
|DATA AND DOWNLOADS|
Download the files for the "Aerial Analyser" here by right clicking on the appropriate link.ORIGINAL ARTICLE
Loading the Picaxe 28X2 with code is only necessary if you are scratch building, or have managed to static zap a chip. The kit from Adelaide Hills Amateur Radio Society (see above) contains a preprogammed chip
The code for the PICAXE 28X2 was prepared using the Programme Editor from Revolution Education and is in BASIC format. This simple FREE program will program and modify the code in any PICAXE chip. and is either known as the "Programming Editor" or by its file name of BAS805.exe. Use the following link to get it........Revolution Education
It is firmly recommended that a standard serial port is used to load the Picaxe chip. If you must travel the unreliable USB route, then make sure that you buy the programming cable from Revolution Education, or at least purchase a conversion cable with a genuine FTDI chipset. Many (most) USB to serial cables on the market use counterfeit Chinese chipsets which do not fully implement the conversion between USB and serial standards, and/or have dodgy driver software. Another and probably cheaper way is to buy a genuine serial card for your desktop.
(a) If you are amongst the first fifty kit purchasers, please note that the LCD contrast trimpot. correctly labelled P2 on the circuit board, is incorrectly labelled P7 in the circuit drawings.
(b) Effective December 16th 2015. Amongst the capacitors currently supplied in the AHARS kit are two monolithic multiplate 1 nF capacitors (labelled 102) with "cranked" leads. Straighten out both leads on these capacitors before installation so that the body of the capacitor sits right down hard on the printed circuit board surface. Failure to do this may cause the oscillator to stop oscillation at around 50-55MHz.
(c) Removing resistors and capacitors from the PCB without damaging it-- Physically cut the component in half with sidecutters. This allows you to remove each component lead separately with tweezers without stressing either the pads on either side of the board and/or the plating in the hole. Heat the pad on ground plane side of the board if one side of the component is earthed. The pad on the other side of the pcb will rapidly come up to temperature. Remove the solder in the plated through hole with either solder wick or a solder sucker. Install the new component.
(d) The supplied meter 91C4 meter only needs to have the revised scale attached as described. The pcb “TEXT” position is bridged with a shorting link. ( Courtesy VK5FVSV :-))
(e) Lack of an engraved front panel-- Unlike the previous HF only analyser, this unit is not supplied with an engraved front panel, but with a flexible stick on label which has been colour laser printed on to plastic film. This label is quite durable and has the following advantages (1) It drops the kit price dramatically - which is very important to those who build the kit in less fortunate nations than Australia. (2) It allows the use of any LCD (including the ultra cheap ones on Ebay and AliBaba) without having to retool to accomodate the widely different face plate sizes found on LCDs from various makers. (3) It can be produced cheaply at home, which is singularly important to those who mess up during assembly of the unit. :-)
(f) Colour codes for the rf chokes- In the club kit, all inductors are packed in a separate envelope to the resistors so they can be easily identified. Note that band 4 ( the tolerance band) on these inductors can be either silver or gold depending on what AHARS manages to purchase, but is usually silver.
Here are the colour codes..........
47uH= yellow, purple, black, band 4,.....33uH= orange, orange, black, band 4,.....15uH= brown, green, black, band 4,
3.3uH= orange, orange, gold, band 4,.....820nH= grey, red, silver, band 4,.....270nH= red, purple, silver, band 4
(g) On the second highest frequency range (50-96MHz), some builders are finding that the lowest frequency obtainable is around 52MHz. If you want coverage down to 50MHz, simply squeeze the 5 turn coil along its length to bunch up the turns and make it shorter. Alternatively, if you want coverage lower than this for say RC model development, neatly wind 120mm of 0.5mm diameter enamelled copper wire on to the body of a half watt 1 megohm resistor to make an inductor, and replace the 5 turn coil with this assembly. This will give you coverage down to around 44MHz.
(h) Identifying Pin 1 on an IC...... Pin 1 is normally marked in one of three ways-- a dot moulded into the plastic next to pin 1, a rectangular indentation moulded into plastic in the middle of the pin 1 end of the IC, or a painted line next to the labelling left hand side and across the width of the IC at the pin 1 end. If an IC has none of these marks, then simply turn it until the labelling can be read and is horizontal. Pin 1 is then to be found on the far left of the lower line of pins.
(i) Note that at VHF, the analog meter display of magnitude of total impedance is always going to be more accurate than the LCD figures (at impedances other than 50 ohms), simply because there are less strays in this simple circuit, and also because there is no multiplication of errors during calculations in the microprocessor
(j) Hard to get inductors - scratch builders may have considerable difficulty in acquiring two of the values of RF choke used. These are L7- 270nH and L4- 820nH. Experiments have shown that L7 can be replaced by the common value of 1uH and that L4 can be easily fabricated from two standard values, 1uH and 4.7uH in parallel
(k) Inability to set full scale on the analog meter. As at mid July 2016, over 250 analysers have been built and it turns out that not all BFR96 transistors are created equal. Five analysers have now given trouble, due to the output stage TR5 breaking into oscillation at over 6GHz with the AHARS supplied BFR96 units. This oscillation cannot be seen on any common test gear (cro or spectrum analyser) and the output sine wave looks nearly normal, apart from the fact that the positive half of the cycle is slightly smaller than the negative half. Also, on a typical 50MHz CRO, the waveform has a small "hook" on the positive going slope of the sine wave just after the slope crosses the waveform vertical centre. Oscillation is also indicated by the fact that with no load, and on the lowest frequency range, TP5 will be at around 2 volts dc instead of 60 millivolts or less, and TP3 will be around 2.5 -3.5 volts dc. The quickest fix is to fit the analyser with a genuine Philips or Siemens BFR96, but there are some other alternatives too, one of which is to completely remove C14, the 100pf capacitor in the emitter of TR5. It seems that the BFR96 units supplied with the kit have higher gains and larger Fts than the transistors used in the prototypes. If removing the 100pF does not completely solve the problem, connect a gimmick capacitor of around 0.15pf between collector and emitter of the BFR96. Use a standard 0.25 watt 100K metal film resistor for this purpose, which has this stray capacitance between it's two brass end caps.
(l) Incorrect frequency displayed between 25 and 30 MHz on 13.4 to 31MHz range only. See this pdf file on how to fix this firmware bug.......Firmware Fix.pdf. All kits sold by Adelaide Hills Amateur Radio Society since 30th August 2016 have the new software loaded into the microprocessor. Only a very small mod. to the pcb is required.