137MHz Turnstile Antenna for APT satellite reception


View of the obstructing chimneyAn early incarnation of my homebrew turnstile

Early versions
An early incarnation of my homebrew turnstile at my previous home. It is mounted below the 2m collinear antenna I used with my UIView APRS IGATE and I added a separate vertical dipole as the diversity antenna. Unfortunately verticals seem very prone to terrestrial noise pickup and the pager tower although several miles from this location still caused problems although obviously not as bad as I get at my current location. I ended up only only using the turnstile. A further problem was the location – the large chimney is due north of the antenna and I believe it cause some obstruction of satellites at low elevations to the north.

Homebrew 137MHz turnstile antenna for reception of APT weather satellite picturesDesign
The picture on the left shows the fourth physical incarnation of essentially the same antenna – A pair of two element yagis mounted at right angles and fed 90 degrees out of phase using a quarter wave of coax. The design was optimised on polar diagram and gain.

Two versions were constructed from 8mm microbore central heating pipe and 40mm waste pipe. The other two (including the one pictured) used copper wires twisted together using an electric drill. Twisted wire elements tend to droop a little as can be seen but this doesn’t seem to seriously affect performance.

Construction Notes
These notes are brief as there is nothing especially taxing with the construction…
It is advisable to fit a short length of plastic rod between the dipole halves when using 8mm tube and a short length of plastic tube with a cutout for soldering the connections over the twisted wire in those versions.

A “dirty balun” should be formed by taking the coax through the tube wrapping it around 5 times and then taking it inside to make the connections. The necessary holes should be marked and drilled before connecting the coax.

Connections for RH circular polarization

To achieve the necessary right hand circular polarization one dipole of the pair is fed through a quarter wave of coax from the feed point of the other. This diagram should make plain where the inners need to go. Getting this wrong will result in left hand circular polarisation and an antenna that doesn’t work. I’ve received a comment below pointing to a website giving contradictary information on the phasing for RHCP. All I can say is that the above connections worked for me when I was using the turnstile. Maybe if I had reversed them it would have been better – I really can’t say and as Wikipedia says there are two contradictory definitions for RHCP I think my advice now has to be – suck it and see.

Dipole: 99cm (8mm tube) 100cm (twisted wire)
Reflector: 106cm
Spacing: 39cm
1/4 wave delay line – I used 75ohm TV Coax and using the MFJ analyser the length worked out to 39.5cm.

Here are the EZNec plots of a two element yagi with these dimensions. Lack of skill and laziness mean I haven’t produced a plot for the complete turnstile.

I received the following email from Rodney Byne and have added the picture he sent of his own turnstile.

Turnstile for APT satellete reception by G7OEL

Greetings Mark from Rodney (retd) G7OEL,

I just stumbled on your site as I was looking for qfh v turnstile user comparisons.

I built an RX2 kit and turnstile in 1996 and had the foresight to wind a 12 turn feeder balun before connecting to the junction box. The aerial survived three house job moves – Weston super Mare, Coventry, Scunthorpe and no pager interference bars were seen at any of them, or am I just lucky.

Resultant pictures are marvellous and always have been. So I don’t think I would want to try a qfh, for me there’s no point.

Keep up the good web site work and 73,
From Rodney.

  8 Responses to “137MHz Turnstile Antenna for APT satellite reception”

  1. I’ve used a turnstile for 6m. with the dipoles fed out of phase using 75 ohm TV co-ax at half a wavelength high for the last 4 seasons and have had very good results. The SWR meter says it is 1.3:1 I can generally work everything I can hear, Last season I managed KP2/K3TEJ with good signals both ways, it is also fairly good with difficult close in distances – GM,GU and EI in my case, I’m located near Sheffield in the North Midlands of the country. A few tips I’d offer is that you can waste quite a lot of co-ax getting the length of phasing line to the correct length so get hold of extra before you start! Also. make really sure the phasing line is joined to the antenna very firmly and it will stay together in the wind. Don’t have any cheap switch in the antenna system for 6m. as the losses are substantial use a really good low loss reliable one. (I don’t have any.)

  2. Wave propagation between two identical antennas of the same circular polarization is analogous to being able to thread a nut from one bolt to an identical opposite facing bolt.

    The bolts screw in the same direction (right hand in this case), however relative to each bolt, it is opposite directions, which is why your connections for the turnstile work.

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