Turnstile Modifications

[SGAARC]

Please help!

Bit of a random question and despite my callsign I don't know the answer, hence asking!

We are using a turnstile antenna designed for weather satellites for the FUNcube satellite. Here it is http://www.moonraker.eu/turnstile-137-weather-satellite-antenna-1

Can we simply shorten the top dipoles to make it resonant for the 2m band? Obviously there is a relationship between the upper and lower elements both in length and distance apart but I wasn't sure how critical it is. Would it make enough of a difference for receiving FUNcube satellite signals to justify taking it down to modify it?

Also, once tuned, if it can be would it then be any use for transmitting? This would then allow us to use the 2m band for local contacts without the need for a separate antenna. That said would it perform too poorly compared to say a dual band colinear?

Hope that all makes some sense.

Paul
M0HNF

[wa8sme]

Hi Paul,

You might consider one of the many homebrew antenna designs that have been published over the years. This is just one example: http://www.arrl.org/files/file/ETP/ISS% ... rticle.pdf

You could certainly adjust the element lengths of the antenna you are currently using. But I suspect that any change for receiving wouldn't have much effect on your success with receiving FUNCube. In all likelihood, make the element adjustments without changing the length of the phasing lines would probably have more of a negative effect than just using the antenna the way it is.

These antennas are simple projects to duplicate, the hardest part is getting the accurate length for the phasing line and the transformer lines (you can't always trust the velocity factors published for particular coax).

73,
Mark WA8SME

[SGAARC]

Hi Mark

Thank you for the info. Could be a future project for our pupils to look at.

After chatting to a friend of mine on the radio last night and your comments I think the way forward is going to be - leave the turnstile as it is and push for an additional antenna to cover 2m and 70cm. That way we kind of get the best of worlds so to speak rather than a compromise.

There is also a chance that we can have both antennas mounted on the main school building. Giving a clear view of the horizon in all directions. So that will improve satellite reception and 2m contacts.

Regards

Paul
M0HNF

[SGAARC]

And for my next daft question...

Would it be possible to mount the turnstile below a dual band colinear on the same mast? Could the aluminium mast affect the antennas performance? If so what would the ideal vertical separation be?

I only have one chance to get it right if we get permission to mount them on the main school building!

Paul
M0HNF

[wa8sme]

Hi Paul,

I would think that it would be possible to mount the co-linear antenna on the same mast above the turn-style without any major degradation. The co-linear antenna basically stops at the mounting point, the turn-style elements are at 90 degrees to the mast and below the co-linear, so the interaction between the antennas should be minimal. As with any antenna installation, it is all a mater of trade-offs; what you can afford, and what you are allowed to install, and what you can safely install, and in the case of a home installation, what the XYL will allow (which is probably the most important factor). So you do what you can do and hope for the best. I believe what you envision will work well for you across your operating spectrum. That is my two cents.

When I was a school principal, I had it easy. I installed what I wanted to install on the weekends. If there was a complaint, it went into my round file. I just had to make sure the roof didn't leak on the freshly polished gym floor...you didn't ever want to irritate the school custodians!

73,
Mark WA8SME

[SGAARC]

Hi Mark

Thank you again for your help.

My next hurdle is securing the funding! Not sure it will stretch to us using RG213 so may have compromise on RG58M for the feeder. The distance from the antennas to the radios is approximately 25m so I guess we will lose any gain that the colinear gives us! On the plus side the extra height and clear views will improve performance. As you said it is all about making the best use of available resources.

Will let you know how it all goes. Probably won't be completed until the summer break.

Regards

Paul
M0HNF

[g0mjw]

If its 25m, ideally you need a pre-amp.

With RG213 for a 25m run you will have 1.8dB of cable loss, which is just about bearable. With RG58 it will be 4.5 dB which is a huge loss. With Heliax - e.g. LDF 4-50 it would be 0.7dB which shows the value of a good cable but is generally expensive unless obtained surplus. With a mast head pre-amp you no longer care about the cable loss, within limits.

If you are going to all the effort of putting the antenna up, I really would recommend using good coax. RG213 will cost £1.70 per metre from Modern Radio in Bolton so its not that bad. Any cheaper, e.g. Ebay and the chances are it won't be very good, or if you want convenience you can pay £3.99 per metre for the same stuff from Maplin.

Mike