Ground antenna recommendations

[Josh Atkins]

Hi,

I'm currently finishing a payload to hopefully fly sometime in the next few months (Covid permitting) and am looking for some advice on ground antennas.

I have the Aliunce 70cm yagi recommended on a recent thread for receiving rtty, but I also plan to fly an Uputronics Lora module (868 MHz) and would be very grateful for any recommendations people can give for receiving. I'm using packet resending for SSDV (it's a custom tracker transmitting WebP images instead of JPEG, so the bitstream has to be reconstructed exactly) and receiving with an identical 868 Mhz Lora module.

I'm also looking for the right mag mount antenna for receiving rtty in the car. I came across this and this; any suggestions would be greatly appreciated.

Regards,
Josh

[John Laidler]

The problem with 868MHz LoRa is you might be on your own as I don't think there are many stations using that frequency, unlike 434Mhz. I may be wrong and it will depend where you are launching from.

As you will know you can get 868MHz Yagis but how many Yagis can you wave about at the same time?

My instinct would be to use a straight wire mag mount cut down to half a wavelength - 164mm.

John

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[Josh Atkins]

Thanks, I will have a search for the right mag mount. I currently have the stubby antennas from Uputronics for 868 MHz but assume these won't get range during a flight (bought for testing).

I'm planning to launch from Elsworth.

Josh

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[Steve]

For fixed station use I'd recommend something like a co-linear - or even a simple ground plane with sloping radials:

    https://www.nutsvolts.com/uploads/wygwam/hww-figure-1.jpg

For mobile station a mag-mount 1/4 wave seems to work fine - I use one of these cut for 434MHz:

    https://www.radioworld.co.uk/tx10_deluxe_taxi_mag_antenna_with_bnc_plug_fitted

and one of these cut for 868MHz:

        https://www.ebay.co.uk/itm/231868265488

    Steve

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[Antal Vincz]

Hi,

I recommend this tape measure yagi for 70cm band. Look at the attachment.
If you use the tape measure to build the elements , the antenna will be foldable.
If you use to antenna elements aluminium or copper tube or flat line, the antenna is very stable and good for base station.

https://www.ebay.co.uk/itm/193929238273?var=494129137835&hash=item2d27155701:g:iE8AAOSwajtgPkKf
https://www.diy.com/departments/aluminium-flat-bar-l-1000mm-w-10mm-t-2mm/254250_BQ.prd

For the antenna boom good any plastic or wood rod.

Type	Length [mm]	Position [mm]
Director 4	260	25
Director 3	275	185
Director 2	290	297
Director 1	300	410
Dipole	323	490
Reflector	342	606

I'm recommending a very simple antenna rotor for balloon tracking.

https://sarcnet.org/rotator-mk1.html

This antenna and the rotor good for low orbit satellite tracking as well.

Regards

Tony

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[John Laidler]

I've been playing with the 70cm folding Yagi today using a new to me gadget called a NanoVNA.  This is a small vector network analyser which cost me about £40.  You can use it on its own but it is easier to connect it to a PC and run the program NanoVNA Saver.  The results from the folding yagi were initially mixed until I noticed that a nut near the antenna connection was loose.  After tightening it up things became a bit more stable. You can see in the enclosed photograph that the adjustable bar on the antenna needed to be as close to the boom as possible.  It could go closer if I cut some of the plastic sleeve off but it is good enough where it is I think. The rogue nut can also be seen just below.

The screenshot shows what NanoVNA found.  The antenna is resonant at about 437 MHz but the curve is very flat and the SWR is acceptable at 433 MHz I think. The tests were done with me holding the end of the boom in my hand in a small room, it would be different mounted on a mast in the open air I suspect but as the intention of this antenna is to use it handheld I think the test was reasonable. 

If anyone wants to buy a NanoVNA there are several suppliers and differing prices. I bought mine off Amazon just for convenience, it may be cheaper elsewhere. You need a set which includes the two short cables and most importantly the calibration items, which are three SMA plugs for short, open and 50 Ohm tests. You need these to do to calibrate the NanoVNA. Some sets do not have them.

I watched two videos made by someone called the "Thesmokinape" on YouTube which were helpful.  One is an introduction to the NanoVNA and the other  shows how to use the NanoVNA Saver program. Google will find you NanoVNA Saver on GitHub.

John

M0WIV   (my new callsign as of 7 days ago!")

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[Antal Vincz]

Hi John,

Congratulation you new callsign! Best wish to you.

Your antenna measurment is correct.

Yust one remark if anybody maesuring any antenna inside at room.
The human body is a huge capacity.
If the body is close to the measured ant., that is possible modify the result.

And important, all the time calibrating the VNA before use it.

73' & DX

Tony
M0GLU

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[Josh Atkins]

Thank you John, Steve and Antal for your suggestions and advice, it is all very helpful. I am currently waiting for some RG174 coax to arrive and will then try making a payload antenna and see how the yagi performs. My tracker uses some custom software  (WebP rather than JPEG) so in terms of receiving images I am on my own although I may make some receiving code publicly available.

Josh