VHF/UHF/uWave - different prop over the same path

[VK4CZ]

During the JMFD an interesting phenomenon was observed. Then and now, I'm stumped to explain why.

Contacts between VK4CZ/P at Gus Beutel Lookout (QG62cp) and VK4GHZ/P at Mt Richmond (QG61db) were attempted on 144, 432, 1,296, 2,403 & 10,368MHz over the 175km path.

Prior to the weekend path analysis was run, and we knew that it wasn't line of sight, but offered minimal (compared to many other paths in south east VK4) obstruction.

We had great success on 144, 432, 2,304 & 10,368MHz (albeit a one way RX report to VK4GHZ/P.... tones from VK4CZ/P were heard by VK4GHZ/P but we were unable to complete on SSB).

However, we completely failed to complete on 1,296MHz. In fact not a trace heard in either location. Nil, nada nix! Yet both stations had completed a number of contacts on 1,296MHz with other stations with RX reports well over RS59 on paths up to 275km.

The station capabilities were:

And the path:

Path analysis completed for each band, just reinforced the expected free space path loss and fade margin changed by band.

Weather conditions over the four hours remained consistent (just a sensational VK4 autumn day) as you can see in this pic of the view and path from Gus Beutel Lookout (VK4GHZ/P was directly over the dead tree in the foreground).

So is there a explainable reason, or just the vagaries of UHF & uWave prop!

I'd appreciate your thoughts? Even if just to help our ongoing learning.

[VK3AMZ]

Hi Scott.....I'm not overlay familiar any with the details of the path but I can see that there is an obvious problem with obstructions.

My first question would be ....Was the gear working ? (sorry but that's the most obvious)

Secondly, because the path is obscured then we have to take into account factors of Fresnel zones and multipath.

If the 1296 MHz antenna had been moved only a relatively short distance from where it was planted then the results may have been very much different.

Due to a coherence of the signals, that same signal arriving 180 degrees out of phase (or multiples) would cause total cancellation of the received signal (at either end). This means a signal of some comparable strength reflected but delay by a distance of lambda/2 (or multiples) would cancel the "primary" signal.

I'm sure you go mobile on VHF/UHF......all that flutter on the signal is not only the received signal strength varying it's also due to phase cancellation due to multipath (Fresnel effect)

For a vehicle in motion at a fixed speed, the flutter rate increases with frequency.....Why shorter wavelength.....that's that lambda/2 thing again.

Next time you strike something like that ...move the antenna it should make a difference

73's

If you are still not convinced think about how a balanced modulator/mixer works!

[VK2OMD]

During the JMFD an interesting phenomenon was observed. Then and now, I'm stumped to explain why.
...

Though you have given quite a bit of data, the number of elements on an antenna isn't directly useful.

I have used the SWAG metthod (Scientific Wild A**** Guess) to look at the effects of tx power and antenna gain in one direction only.

Above are models of a free space path with parameters broadly like you have suggested.

The rx power on 1296 is some 10dB lower than 432.

You could try the same thing with better estimates for antenna gain.

I left out feed line losses (a bit pointless when I could be a dB or two out in gain), and there is the question of realistic antenna gain figures, but -60dBm for a free space path should be pretty usable for SSB telephony. Add an obstruction, and you might wipe 30dB off that but still pretty good... though performance on the higher bands suggests the obstruction was not too serious.

Fresnell effects might account for some of the problem.

I would be asking the question, was the gear at both ends working to expectation. People do silly things on field days some times! Has the gear checked out since?

Owen

[VK4CZ]

Thanks Arie and Owen.....

Was the gear working to expectations?

Whilst there is past video proof of me leaving my IC-910H with ATT on (when co-located with VK4EA), in this case I believe that we have good reason to think that the stations at both ends of the path were working to a level that would have been acceptable...

both stations had completed a number of contacts on 1,296MHz with other stations with RX reports well over RS59 on paths up to 275km.

Was this

Due to a coherence of the signal

, quite possibly.

As we didn't get a chance to make changes to the antenna positions, I consider this a very real possibility... only this morning sitting in traffic I was 'playing' with the coherence of signals from a local 2m repeater by rocking the 'truck' back and forth about 1/2wl (I sat for 20min in traffic and only moved 2 car lengths ) to see the signal go from S4 to nil (apologies to David VK4DC who was trying to talk to me at the time and I was trying to find the 'sweet spot' to hear him )

Given we had levels of success on other bands I had expected that the observed impact on 1,296 would not have been as significant as it was, but we may have been highly successful in our antenna positioning.

Appreciate the thoughts.

Put this down to another great learning experience as part of AR.

[VK2XSO]

Not SSB, but our local FM repeaters (VK2RTM) on 2m, 70cm, 23cm offer a little window into local propagation.
Each repeater has a very similar coverage locally, so it's interesting to see that quite often only 2m will open to DX.
Sometimes it's good enough that 70cm will open too.
I've yet to see 23cm open.

The 23cm antennas are in the same positions as the 2m repeater antennas.
The 23cm repeater has a better performance over the same LX paths. Where only 25W with a 4.5dBi antenna for 2m on the fringe of the coverage area works, 1W on a rubber ducky works on 23cm.
Inside the coverage area, 2m continues to work behind small hill. On 23cm the same radio horizon is a lot sharper.

[ZL1TPH]

Hi Scott, three cents worth from ZL ..
Arie VK3AMZ made a very good point, we had a 50 m/Watt 23 cm beacon here in Auckland, only 70 odd km's away, over a ridge line from my QTH and I worked out how best to RX it, by moving the loop Yagi to the other end of my 6 metre wide garage. In other words I worked out the best spot to receive this low power beacon.
Thinking aloud and maybe the reason why 23 cm was down, running 10 Watts on 23 cm is a hard slog at the best of times, best to get the power up to around 80 to 100 Watts and then things start to perform, just that the extra 10 dB makes a real big difference and if I recall years ago with my 10 Watt 2 metre rig, once fired up my 6/40 tube (for a 100 Watts) made the same difference, in a big way ...
Cheers, Steve

[VK4CZ]

Steve

I can't disagree with Arie, Owen and your observations and as a result accept that if both stations could have either moved the antenna or increased power we could have achieved a completed Q.

But given the success achieved on bands either 'side' of 1296MHz, and that both stations completed numerous Qs on 1296 over different and longer paths, the probability was that a variety of factors were in play and it may have taken more than one change to 'open the path'.

When operating portable in a contest, and in my case just for a few hours completely by myself, and with only the gear I brought, the options to add, change or try new things is limited. And as you'd appreciate from you're portable operating (which we remain ever grateful for), that all to often you find yourself as busy as a one armed paper hanger and moving an antenna is at that time a last resort.

For me however the learning from this has been that creating some flexibility in the antenna mounting so that at least I can control the antenna placement will in future be a higher priority for me.

Thanks all for the feedback.

[VK3AMZ]

G'day again Steve..........

There are a plethora of things at play here but I tend to reduce a problem to it's lowest common denominator, which in a nutshell means the most likely cause.

This "frequency" selective path effect is not a new phenomena and is a critical parameter in any link or radio path design. I myself experience with a local 23cm beacon (coincidentally!) an obscure path issue . The direct path is rubbish, (antenna pointed at source) but if I point the antenna to the North West, it gives me a much better signal and a more definitive peak (the antenna is working within spec.). I don't have line of sight to the beacon but I do have hills/mountains that seem to provide a way around the obscured direct path.

That would be food for thought in the future if you come across the same effect again, rotate the antenna in a direction that would look to an observer ridiculous considering the direction of the source. You will be very surprised what may happen!

Good Luck......73's Arie

[VK3AMZ]

Sorry Scott about your name......... I was in dream world .......

[VK4CZ]

Been called worse Arie