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.

Construction
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

Polarisation
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.

Dimensions
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.

Spelling is a lossed art.


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

  1. How did you get the spacing between reflectors and dipols? I got about 31 cm for spacing (0.15lamda).

  2. I used EZNec software to model the antenna and trial and error of the spacing gave me the result I wanted – highest overall gain and least low angle null – as explained in the EZNec plots page linked to above. Whether or not the software design matched reality I really don’t know as I have no means of testing it.
    Mark

  3. Hi, I’m trying to understand the whole RHCP and LHCP and I’m a little confused. Your website and http://www.qsl.net/s/sv1bsx//antenna-pol/polarization.html contradict one another. Is there something I am missing or has one of you made a mistake?

  4. To my previous comment, I meant in terms of your diagram, looking from the bottom of the antenna upwards vs their rear view of yagi dipoles

  5. Hmmm…

    truth be told I’m uncertain now. When I built my first turnstile I used http://www.ve3sqb.com/hamaerials/oz2oe/build_yagi.html as the reference for getting the polarisation correct. Checking Wikipedia it seems that there are two contradictory conventions in use – http://en.wikipedia.org/wiki/Circular_polarization#Left.2Fright_handedness_conventions All I can do is say the using my connections as shown produced an antenna that worked for me. Maybe if I’d reversed them it would have been a lot better. Comments from others are most welcome.

    Mark

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