Long Marston Test 17 October

[JCurtis]

2. is not practical. 30dB is an increase in power of 1000 times. The ARF runs, it is said, 500mW, so we would need 500W to do it this way. Quite apart from licensing issues, that sort of power is a) dangerous (think microwaves again), b) requires a lot of kit to produce it and c) would need significant battery/generator capacity to support it.

All very true, but the ARF runs at circa 90mW, it cannot produce any more, the reference design from TI recommends a maximum of +18dB, while the radio toolset (RFStudio) used to set them up tops out at +20dB as an available setting. It could well be less depending on the power supply, cables, antenna etc.

Numbers still not great, but with closer to +27dB at the source and a receive sensitivity of circa -107dB that some other potential packet radio modules have there is scope for improvement.

[The Westmorland Flyer]

Good points Jeremy, thanks. I thought the ARF could run 500mW on 869.4MHz so my apologies for assigning it higher performance that it deserves.

When you say +18dB, I assume you mean +18dBm (since dB is a relative term). 18dBm is 63mW, 20dBm is 100mW. Even 27dBm, aka 500mW - the maximum legal power in the ISM band - is only going to offset 9dB of losses, which in the context of antenna gain/loss and, especially, in terms of shielding losses is not much, sadly. I don't know what the ARF receiver sensitivity is but I'd guess it's within a few dB of -107dBm, so not a lot of room for improvement there either.

I fear that any solution that does not include external antennas is always going to disappoint.

[JCurtis]

Good points Jeremy, thanks. I thought the ARF could run 500mW on 869.4MHz so my apologies for assigning it higher performance that it deserves.

When you say +18dB, I assume you mean +18dBm (since dB is a relative term). 18dBm is 63mW, 20dBm is 100mW. Even 27dBm, aka 500mW - the maximum legal power in the ISM band - is only going to offset 9dB of losses, which in the context of antenna gain/loss and, especially, in terms of shielding losses is not much, sadly. I don't know what the ARF receiver sensitivity is but I'd guess it's within a few dB of -107dBm, so not a lot of room for improvement there either.

I fear that any solution that does not include external antennas is always going to disappoint.

Yes, that should all be dBm.  The TI reference design for the chipset used in the ARF is -107dB, as the booster IC also has an RX amp.  The ARF follows this reference to an extent, but has been altered to fit the form factor.  There is no data sheet for the ARF, so the actual details are unknown.

[Richard W]

All the points about antenna location are well made.  However, I think that something more fundamental is going wrong here.  Recalling my experience yesterday, I had good LOS through the flimsy windshield of a Skyranger to a ground station, from a height of about 2000'.  I was in the region for about 10 minutes, and would therefore have transmitted about 300 packets.  The ground station received only 6, and I received only 12 from him  Simultaneously a ground station 4 miles away could receive me without difficulty (but I never received anything from him).  According to my understanding, the inverse square law says that the signal level at the local ground station should have been about 24 db greater than the one 4 miles away. It seems to me that there is a problem with the receivers, or maybe frequency drift?

[The Westmorland Flyer]

I had good LOS through the flimsy windshield of a Skyranger to a ground station, from a height of about 2000'.  I was in the region for about 10 minutes, and would therefore have transmitted about 300 packets.  The ground station received only 6, and I received only 12 from him  Simultaneously a ground station 4 miles away could receive me without difficulty (but I never received anything from him).

If I understand this correctly you had two ground stations, one 2000ft directly below you that struggled and the other four miles away that worked better. If that is correct then it brings into play another aspect of  simple whip antennas, which is that they do not work equally well in all directions. Notably, for this case, the antenna will not radiate in its own plane, i.e. along the line of the wire. The radiation pattern of a vertical antenna is rather like a ring doughnut plonked flat around the antenna - lots of signal  horizontally through 360 degrees but nothing above or below. Effectively radiation is perpendicular to the antenna wire. The nulls off the ends are often very sharp indeed - a characteristic that is used amongst other things for direction finding.

It is harder to explain why the station four miles away could see you but you could not see it. Don't have an opinion on that at the moment but I think it highly unlikely that frequency drift is an issue - these units are quite broad band after all.

[Paul_Sengupta]

Yes, definitely something more going on than RF line of sight/obstructions/antenna placement. In the tests CC and I did, one unit could see another unit at 5 miles. The reciprocal unit couldn't see the first one until 40 yards away.

On the ground, we verified this with two units sited together...the one unit dropped out at 40 yards, another at 300 yards.

If we were actually getting 68mW/100mW all the time and everything worked as it should, we'd at least get a mile's notice, and up to 5 miles in good line of sight. I guess we'd get even more with fully working units at 500mW.

[Moffrestorer]

Paul,

There must be some difference in the "build" of the two units that could be investigated. Have you tried Ian Fallons suggestion earlier in this thread of measuring power by way of Master /Slave working?

[Paul_Sengupta]

Not yet. On the cards!

[Moffrestorer]

I think we were getting a bit more during our air to air tests at Long Marston. Subject to me solving another curiosity, perhaps a lot more. I wouldn't write the ARF off just yet.

Hi Ian,

I'm intrigued, as to this curiosity? Any conclusions as yet?

[ianfallon]

Reluctant to commit to this but it does look like my PAW was being picked up overhead Shotteswell by Bryan's near overhead Long Marston over ARF which by my reckoning is about 14nm / 25km

[Richard W]

If that is correct then it brings into play another aspect of  simple whip antennas, which is that they do not work equally well in all directions

I approached station A (the near one) from two miles away, from the direction in which it had the best view of the sky.  So the slant angle would be not much different than that to station B.  I got no response until I was nearly overhead, so I think that rules out the radiation pattern.

A thought occurs to me, do you think that the receivers are being swamped by noise radiated by the Pi, especially in Slice of POD examples, like mine.  The ARF is closer to the noisy stuff than the earlier glue pen models.

I am going to do some master/slave tests when I get my second ARF, which I persuaded J Curtis to sell me 

[captchaos]

btw do you know about the master / slave test mode ? You can put one in each mode via the Configuration web interface and then the logging will show power levels. Would be interested to see the values you get.

Not sure how to do that. I tried and got ARF logging messages on the master like this but have no idea what it means!

aSSRSSIS-58 aMMRSSIM-54

[Ian Melville]

Model aircraft use 100mW on the 2.4Ghz band and have known control issues if a person get between the pilot and model, and that is at less than a kilometre range. Not surprised that internal and shielded antenna reception is poor between PAW units, not to mention poor antenna setups.

We seem to have two threads running on this issue?

[The Westmorland Flyer]

2.4GHz is absorbed by water-based things such as foliage and people significantly more than is 800MHz, so I am not particularly surprised. It's the reason that 2.4GHz is used for microwaves. 800MHz is actually a rather useful frequency because it is relatively unaffected by water but even 800MHz won't go through metal unfortunately!

[stephenmelody]

Just as a complete amateur, this may be a stupid question, but would the tv tuner interfere with the ARF at all?