Aerial issues:

[sblair1290]

Ok - our little school HAB club has managed to successfully decode a signal across the classroom. The boys took the kit out at the weekend and discovered the range (without an antennae) was around 10m. We have a HAB preamp and will hook that up as soon as the adapter arrives to couple it to our Funcube dongle.

Q1: what are the main concerns regarding aerials. There is relatively little advice on this when compared to other areas. I assume this is because it is a simple issue to cater for!! We have used a quarter wavelength piece of single thread wire (insulated) stuck in the dongle (as suggested in the Pits manual). This did increase the signal but not the range (if the boys carried out the test correctly).

The aerial attached to the pits board is as supplied. This is much longer than the 17cm theoretical length for best results. The length is apparently to allow for the size of the casing. Q2: Assuming the casing have no affect on the aerial output signal strength, what should we do to get best results with this piece of aerial?

It seems we have a very long way to go before sending signals successfully on a 30km round trip. The HAB preamp was not listed as essential so we must be doing something wrong with the aerial setup. Do we need to fashion a 'proper' aerial and if so, what IS a 'proper' aerial?

Our kit:

Pits board + PiB V2 + Sense hat communicating with Funcube dongle pro via DL Flidgi (HAB)/Radio sharp

as usual, thanks in advance for any help.

Stuart

[bkm888]

Hmm... Finally something to exercise my amateur radio side of the brain.

Ok, There are a few tricks you can use to get your PITS board the maximum chance of being heard. First, the balloon payload (your pi radio) will perform best if the antenna is oriented as nearly to horizontal as possible. A misconception about single element antennas is that you can increase signal by pointing the tip of the antenna at your receiver. The opposite is actually true. You can see this for yourself if you look at pictures of multi-element beam antennas used for pinpointing a radio signal. While the boom of the multi-element antennas would be pointed at the target, the actual elements of the antenna are perpendicular to the boom.

A little study of antenna theory will reveal that an antenna element radiates its signal outward from, and perpendicular to the radiator. Additionally, the antenna can get increased gain if there is an appropriately sized reflector behind it and parallel with the radiator. The reflector could be something as simple as an aluminum foil coated cardboard disk of the properly calculated diameter. Since this seems to be a class project, I will not tell you what size to make your antenna or your reflector, however the quarter wavelength for both is the perfect starting point.

Next, the antenna for the software defined radio dongle. This is where you can make the most difference in your signal work. The balloon payload is naturally limited by the weight and wind resistance as to how much antenna you can use. So, the best bet is to magnify the signal received at the dongle radio. Now here we have several factors to consider.

First, you will want to make a beam antenna tuned to your balloon transmitter frequency, and attache it to the dongle with a coaxial type cable. The cable needs to be as short as possible and yet be as close as possible to an odd multiple of quarter wavelengths (1x, 3x, 5x,etc.). This odd number tunes the timing of the signal received to reduce signal loss in the antenna system. But beware, there is another factor to consider here as well. High frequencies move through a cable slower than they do in open air. The difference between those speeds is expressed in a term called "velocity factor". For example, RG58u, the most common type of cable for simple transmitters, has an impedence of 50 ohms and a velocity factor of 0.66.

This means that to calculate the proper quarter wavelength of a cable, you need to multiply the original formula result by 0.66 to arrive at the actual cable length. Remember, the fewer odd multiples of 1/4 wave the better the signal on the wire.

Now for the antenna, I would suggest googling the term "fox hunt antenna project". It will point you to many instructables on building a multi-element beam antenna. Then have the class design a portable mount for the antenna (maybe a camera tripod) and have it near the laptop. Just point the antenna in the general direction of the target and you should be able to receive the signal. The further away the target gets the easier is to point the antenna. Additional study of how the signal lobes of a beam antenna would look (if you could see them in the air) will certainly provide a few days of fun calculations and even visibly show how all of this works. After the first draft of your antenna is ready to test, set up your classroom testing experiment again and report back your progress. I think you will find the signal difference to be huge if you make everything up well. Don't be afraid of the calculations and most importantly, when everything else "looks" right but you still have issues, TRUST your math! If you followed the formulas properly, then the problem will most likely be elsewhere. Second guessing your antenna after it is built is a fools errand. Just do the math backwards and forwards before you build and trust your instincts on this. everything else is just connectors, radios, and batteries

A word of caution. Do not be tempted to make the antenna on the balloon payload to anything more than the single element radiator and an appropriate reflector. In this configuration you will reduce the antenna signal toward space (up) and increase the signal toward earth (down). Any additional elements on this antenna would make it too directional and therefore nearly impossible to track as the signal would be pointed which ever direction the wind wanted. With the suggested configuration above, the higher the balloon flies, the wider the cone of the signal would be pointed down to the earth.

I have played with this stuff before with low power radios (cheaper than the RPi setup) and had lots of fun. But then again my brain craves the mathematics involved in making it right.

Have a blast with this and send pictures later. That would be a treat.

BKM888

[bkm888]

One additional thing to consider...

Most of the fun with balloon tracking is what to do when the balloon is out of range (usually in about the first 45 min to an hour). YMMV (Your Mileage May Vary).

There are internet resources for identifying your payload signal and pinning it to a map. There are other radios always listening and posting their findings. Some of these are automated systems and others are by volunteers. Take the time to figure out how you want to gather that information because it will be essential to recovering your RPi later.

BKM888

[daveake]

I disagree about placing the payload aerial horizontally. The payload itself rotates continuously during flight so a horizontal aerial will cause frequent drops in the received signal.

Pretty much every balloon flight uses a vertical quarter-wave aerial with ground plane, with the radiating part of the aerial pointing downwards. I suggest watching Steve Randall's excellent guide on how to make one from the supplied pigtail - https://www.youtube.com/watch?v=xemKyV1H6Xs.

For receiving, the best type of aerial depends on the scenario. For those chasing the balloon in a car, the most convenient is a magmount roof aerial, for example this one: http://www.ebay.co.uk/itm/Mobile-micro- ... Swxp9W3xQS.

Once the payload lands then, depending on how close you can get to the landing position, and the nature of the terrain between it and you, that aerial will most likely get you the landing position from the car. If not, it's useful to have a small Yagi aerial (small because large ones are unwieldy in the car). The one I use is this -http://www.ebay.co.uk/itm/SPECIAL-YAGI- ... SwknJXxwi3.

Dave

[bkm888]

Well, I guess that all depends on "how" you plan to track your payload. If it is intended to be tracked by networked tracking stations, then a cell phone running the app will take care of the chase car, and get you close enough to pull out the yagi antenna and pinpoint the location. No additional mag mount antenna needed.

Making a your own 70cm band yagi is way more interesting and cost effective than buying one on ebay IMHO. The last school hunt I worked on used the yagi at the launch point specifically to see how far it could be tracked from a stationary point by radio alone. A chase team used the cell phone app to follow it to where it dropped into a pond on a friendly farmers ranch. Thinking ahead by one of the students made sure the payload was equipped with flotation

Amazing what 13yr olds can come up with when given creative license.

[daveake]

The receiver network isn't enough to always get a good last airborne position. Sometimes you get lucky and they'll have it down to the ground; usually the last position will be 500m up, but sometimes it will be 4km up. Second you cannot rely on having a 3g connection in the car so you might not even know what that last position was. So, mobile tracking on the chase car is absolutely vital, and a magmount is what you need for that.

I've seen too many teams lose the payloads due to not being able to track in their chase vehicle.

Dave

[sblair1290]

Loads of great radio advice there! thanks......

In terms of making radios I think that would be the ideal learning situation. In this case however, we are going for expedience since we have been doing this for a while and have had to weather many delays....many simply caused by purchasing red tape! I will definitely consider making the aerial a side project for future balloon club projects.

We will begin amending our ground plane aerial this Friday and I am in the process of ordering the magmount aerial and 70cm Yagi. I take Dave's point about being able to track the payload independently and also to have options with regards to the type of aerial. After spending so much time, energy and money the last thing we want is to see the signal drop out at 4km because it drifted into the Cairngorms! We are going to buy less Helium so we can only do one launch but will likely spend the money freed up on better tracking kit.

thanks again,

SB

[sblair1290]

One more thing.....

the Pits manual has a section on Radio's and scanners but doesn't really elaborate on the difference between dongles and these more expensive bits of kit. We purchased the rather more pricey Funcube dongle pro +. I had envisaged that we would use this in the school with a stationary tracking group while the chase car used a radio scanner. Is this the best setup or could we get away with just using the dongle in the chase car?

There is some info on https://ukhas.org.uk/guides:sdr_tracker - but there is no direct comparison between dongles and radio receivers/transceivers. On the one hand this site says ' it is in theory possible to track and listen into a payload using a sub £20 dongle and your PC' but also says 'It is unlikely you will be able to use these devices for tracking a real flight unless you have a very good antenna' .Why bother bench testing with something that will ultimately be inadequate for the actual flight?

The funcube dongle pro + is meant to receive signals from the funcube satellite so I assume it is not in the same category as the cheap tv dongles. Is it however in the same category as the radio receivers?

over to you.....

SB

[daveake]

The cheap (£20 or less) dongles are pretty deaf, and can be overloaded easily if you have strong signals locally. Both of these issues can be fixed by using a filtered pre-amplifier between the aerial and the dongle - see https://store.uputronics.com/index.php? ... duct_id=53.

The better dongles (Funcube Pro, Airspy, SDR Play) are much better, and will give similar (very good) results, comparable with a ham radio receiver.

As I mentioned earlier, it's absolutely vital that you have a decent setup in the chase car. So if you only have 1 receiver, put it there. You don't need a base station - once the balloon is a few hundred metres up then the receiver network will take over. So the important parts of the flight are on the ground before you launch (so you know it's all working) and in the car at the end of the flight.

Do be careful with dongles in chase cars - I've heard of one team where the dongle got snapped off in the USB socket. Probably worth investing in a short USB extension lead to make damage less likely.

Dave