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Homebrewing by GM8UPI


David McAlpin GM8UPI describes an easy-to-build Dual Antenna Switching Unit & has a ‘how to’ on Panel Marking.

Like many amateurs, I have, or shall have, more than one antenna to use on the HF bands. Perhaps a mix of dipoles, doublets, end-feds and Windoms, etc. To enable fast performance comparisons between antennas, both for transmit and receive, I was keen to use my Icom IC-7300 for this purpose. However, no provision is made with this transceiver, or many others, for the use of a second antenna, direct into the radio. An external switching means was therefore required. A mechanical/manual coax switch would fulfil part of my requirement to some extent but would not enable the use of separate antennas for transmit and receive, switching automatically. By using the radio PTT (shorting contact) signal, I designed this simple unit to fulfil my requirements.

The Circuit

The diagram, Fig. 1, shows the final circuit. I did consider using a switching transistor for the relay, so minimising the current required to be taken through the PTT radio connection, but decided this was not necessary in my case, for use with the IC-7300. The current taken by the unit is approximately 50mA, well within the capability of the radio and, I am sure, most other modern transceivers. On testing I found the unit worked as expected. (I did however insert 56kΩ resistors in the LEDs, to reduce the brightness to an acceptable level).


I used a small piece of Veroboard, 40 x 40mm to mount the relay to the base of the case, which made wiring reasonable easy, if not very neat! (see photographs, Figs. 2 and 3) The only other component mounted on this ‘lower’ side, was the relay ‘back EMF’ protection diode D1.

Wiring to the relay pins was made with RG316 coax and leads kept as short as possible.
The finished unit was tested with 100W PEP and CW, on all bands from 160m to 10m, with no problems. (I suspect the power limiting will be all due to the capability of the relay.) No ‘on air’ testing was done at higher frequencies, but at a pinch, it may be suitable up to and beyond 2m.

NanoVNA Testing

Using this marvellous little unit, measurement of VSWR, loss and antenna cross-coupling was made over the frequency range 1.0MHz to 500MHz for both states of the relay operation. The results are shown in Table 1.
N.B. The results above will have been influenced, particularly above 150MHz, by the performance of the dummy loads I used (as required) on the SO239 connectors of the unit.
All components were sourced from within the UK with the exception of the case. This nice, extruded aluminium item came from China, costing £20 inc. P&P. Total material costs were in the region of £40, excluding the decal material for panel marking.

Freq MHz VSWR Thru Loss Cross Coupling
1 to 30 ≤ 1.05:1 ≤ 0.1dB ≥ −40dB
30 to 75 ≤ 1.05:1 ≤ 0.1dB ≥ −30dB
75 to 150 ≤ 1.20:1 ≤ 0.15dB ≥ −25dB
150 to 300 ≤ 1.20:1 ≤1.0dB ≥− 20dB
300 to 500 ≤ 1.40:1 ≤1.5 dB ≥ −20dB

Table One: Measured Characteristics of switch box.

Panel Marking

(I suggest you read fully before commencing your project!)
So many ‘home built’ projects we make these days, while functioning to our requirements, end up looking, well, ‘home built’ to put it crudely. Marking up front panels with lettering for switches, connectors etc, can be done by various methods, but getting a professional looking finish takes time and a bit of, not too difficult to learn, skill.
I use readily available Waterslide Decal A4 inkjet paper, either ‘clear’ or ‘white’ depending on the job, and with care, this method can produce very acceptable results. From experimenting and numerous failures, I have learned that to obtain the best results with decals, a number of vitally important steps require to be adhered to.

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Start your panel design on your laptop/PC before marking/drilling your panel. I use Microsoft PowerPoint to draw out my panel layouts. This app gives you lots of choice in regard to font type, sizes and colours. As a measurement aid to the next stage, draw the outline of your panel on to a sheet of plain A4 printing paper, in pencil. You can mark any fixing holes on to this sheet and, if reusing a previously used panel, any holes/cut-outs that you want to use, or not to use. If holes exist that are not required, these should be drawn on the sheet and will be covered up with your decal, and carefully filled from the rear of the panel with plastic cement or car panel body filler. This of course would be carried out after the final stage of making your decaled panel, when everything has been set, varnished and hardened (see later). 

Now, using your PC and PowerPoint, draw the basic outline shape of your panel using the rectangle shape, with around 3-5mm inside the outer edge. Then do a quick, plain paper printout and check that the dimensions of your outline match your panel with a small inside margin all round. Adjust for any scaling errors and check again. Now you can start to get creative. I find that using font sizes 9pt or larger are fine and I always use Bold and fairly broad outline fonts such as Arial Black or Calibri Bold. I also like to add an outline ‘line’ on my panels, as can be seen in the various photographs. Your choice. I also like to match the background colour to the finish of the panel/case, whether that be black (as I prefer) or any other colour. By using ‘white’ decal paper, and ‘colour fill’ to your outer panel size marker, choose the shade that gives best match. I normally use ‘white’ decal sheets, rather than clear, which allows me to highlight the print as ‘white’, so giving me white lettering on the black, or whatever background colour you choose.

When lettering for switch or connector holes etc, take care to allow for adequate space for fixing nuts, washers or bezels, etc. Too close and you may damage the decal printing when assembling components on the finished panel. When satisfied with your design, I suggest you make a print on plain paper, and use this to mark out (centre punching, drilling, etc) your panel.

Shown in Figs. 4 and 5 are the decal prints for the project described earlier. Note that I have extended the black background area to well beyond the actual panel dimensions, to allow for trimming the decal close to the white outline. This ensures that no white edge from the decal paper shows on the final result.

Set the printer for standard ‘Photo Paper Gloss’ and ‘Best Quality’. Note that large panels, requiring a coloured background, will take a fair amount of your ink! Once printed and checked as OK, the sheet must be allowed to dry thoroughly in a warm, dust free area. I have found from bitter experience that this will take a minimum of 12 hours at between +25°C and +35°C. I usually use our household drying/airing cupboard overnight!

Preparing the Metal Panel and Decal

Once the panel has been drilled, cut-outs made, etc, care should be taken to deburr any holes and ensure the face to take the decal is clean with no edges that will cause the delicate decal to tear. If not already painted, then this should be done with care, in a dust free area. (My preferred paints, varnish, etc. are given at the end.) Again, left to dry thoroughly overnight. The panel should now be given two very, very light coats of varnish, 30 mins between coats. Spray from a distance of about 200mm. I prefer clear gloss but matt gloss is fine. Another overnight drying period is required.

The decal sheet should now be given the same varnish treatment as the panel, and again allowed to dry for a minimum 12 hours in a warm, dust free environment. I cannot emphasis enough how important this dust free precaution is, in ensuring a perfect finished result.

Applying the Decal

While still on the backing sheet, trim the decal carefully with a craft knife or as I prefer, good very sharp scissors. The final size of the decal should be a few millimetres smaller than the panel edge, or just outside your edging line, if you have one. Take care not to touch the decal print itself, to avoid fingerprints. Using a suitable sized dish, with 2 – 3cm of room temperature clean water, immerse your decal completely, using a cotton bud if necessary. Do not use warm/hot water. Time of immersion will be from around ½ min to perhaps 2 mins.
With your panel on a towel, use a small soft artist-type brush (½in flat) to moisten the panel surface with water. Test the decal very carefully in the water, to see if it will slide easily from the backing. Remove backing and decal from water, allow to drain slightly, and by using the wet brush you should be able to carefully slide the decal on to the damp panel. This should require virtually no pressure from the brush. If you find the decal turning at the edges, delicate use of a wooden toothpick will usually allow you to unfurl the offending edge. The decal should virtually float on the panel, allowing final position adjustment, again very carefully using the soft brush. Once in the desired position, use the brush to gently remove water from under the decal, with very light strokes from centre to edge. Use of a small piece of fine sponge, tissue towel, or cotton bud, to remove excess water is helpful. Ensure all air bubbles and moisture are removed from under the decal and no creases have formed. Final strokes of the moistened brush should remove any blemishes.
If the results are not to your liking, then moisten the decal to remove and start again with a new decal! Perhaps some practice decal mounting is in order.

Finishing Off

The decaled panel now needs to dry thoroughly and again overnight is best. I always finish my panels off with a couple of coats of gloss varnish to totally seal the decal, again requiring overnight drying prior to assembly of any components on the panel. Prior to mounting components, you will need to carefully cut out the decal film covering the mounting holes/cut-outs, etc. I use a very sharp scalpel blade for this. Care needs to be taken when assembling, such as front nut fixing items, to the panel. Using plain washers is helpful, and if possible, tighten fixing from the panel rear, without turning the front mounting nut. A touch of super-glue at rear of panel fixing can be useful.
Warning: do not be tempted to skip/shorten any of my suggested drying times. Inkjet inks are notorious for leaching. You could end up with some very strange and unwanted colours! Also do not use an oven of any sort to hasten drying.
Figs. 6 and 7 are a couple of photographs of my project. The unit, described earlier, measures 100 x 100 x 40mm. The front panel, three-position switch, allows for automatic changeover to be enabled or any one of the two antennas to be used for non-switching, conventional transmit and receive.

Suggested/Required Materials for Decals
1. PC with PowerPoint app and inkjet printer.
2. Waterslide, clear & white Decal paper. ( @ £15/10 A4 Sheets.
3. ‘Plastikote Clear Super’ Gloss & Clear varnish. (various online suppliers) @ £5/400ml Spray can.
4. ‘Plastikote’ paints. Undercoats & colours. (various online suppliers) @ £8/400ml Spray can. (If required.)
73 & happy homebrewing.