Monday, January 11, 2010

The Wonder Whip Mk1

I have just completed the first version of what I shall call the G4ILO Wonder Whip. It's a portable HF receiving antenna for the Kenwood TH-F7E and TH-F6A that can also be used for transmit on 2m and 70cm. It allows you to use the radio's wide band receive capability to listen to the HF bands 20m - 10m plus 6m.

The project was not without its complications. In fact it took three attempts to get the antenna to work to my satisfaction. The antenna is a base-loaded short whip with tapping points on the loading coil to resonate the whip on the amateur bands and a fly-lead with a small alligator clip to select the appropriate tap.

I first thought that it would be possible to identify the best tapping point by scraping away some enamel from the coil and touching the fly-lead to it using an insulated handle. The problem is that the bandwidth of such a short antenna is very narrow and the resonant point depends very much on what sort of a ground you have and whether anything - such as my hand - is in close proximity to it. So it proved very difficult to find a tapping point that gave consistent results. What might seem the right point on the antenna analyzer can be way off when using the TH-F7E or the FT-817.

I had to realize that what I was trying to make was a receiving antenna for the TH-F7E that would be used without any ground or counterpoise, and forget about trying to get a good SWR when the antenna was put on my FT-817. I was eventually able - more by luck than calculation - to find tapping points for each of the bands. On HF the antenna should be used with the 48cm telescopic whip fully extended, but it can be fine tuned by retracting sections of the whip until you hear the loudest signals or noise. Retracting the whip provides quite a range of adjustment - enough to allow 12m to be covered using the 15m tap. As this antenna is intended for receive only, getting an exact match is not essential, and you should hear stations without needing to adjust the length of the whip.

The photograph above gives the critical constructional details. The tapping points were created by making a loop of bare wire and twisting the ends together to form a tag to which the wander lead can be clipped when selecting the band in use. Because the tapping points were found by trial and error I wound one continuous coil of enamelled wire and then scraped away a bit of enamel from a turn of the coil next to each tapping point and soldered it using a fine tipped bit. This was a little tricky.

If I was making another antenna to this specification I would install the tapping points first and then wind each section with the indicated number of turns, scrape the enamel and loop the wire round the tapping point before starting the next section. You could even wind each section of coil separately, testing each section for resonance before you start the next one. This would make it easier to adjust the number of turns if the resonant point does not fall exactly in the right place.

The telescopic whip I used is 19 inches (48cm) in length, in other words a quarter wave on 145MHz. In the 2m/70cm position the loading coil is bypassed adding the length of the wander lead to the length of the whip. The top two sections of the whip should be retracted to compensate for this. I have found that when using a hand-held transceiver the best SWR on 2m is achieved with a whip length that is 5 - 7 cm longer than a true quarter wave. This antenna allows you to achieve that.

After testing the antenna, the Wonder Whip was finished off by wrapping the coil in black self-amalgamating tape. This helps protect the coil and gives the antenna an almost professional appearance.

I am quite pleased with this antenna. It is a pity that I cannot make it cover 40m and 80m as well but that would require a considerable number of extra turns on the coil and would make the antenna somewhat unwieldy.

I also fear the plastic pen body is a bit fragile and would be rather easy to break. I broke it in half once by applying too much heat while soldering to one of the tapping points which melted the plastic - fortunately it stuck back together again when the plastic cooled, but I may have weakened it. I would like to make a more robust version if I come across some suitable materials. The current version at least has the advantage of light weight, and does not put too much stress on the SMA connector of the TH-F7E.
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