Author Topic: Twin internal dipole aerials (total 4) with SMA Y pigtails  (Read 1444 times)

neilmurg

Twin internal dipole aerials (total 4) with SMA Y pigtails
« on: March 31, 2021, 07:11:45 pm »
I've been looking at the very useful polar map of my PAw coverage.
My setup has 2 dipoles on the passenger side near the A pillar, it's very close to vertical, unobtrusive and with a great view forward. The PAw aerial is highest so it can look down more effectively
I get some shadowing from the 'ugly bags of mostly water' at my 8 / 9 7 / 8 o'clock.

I'd like to double up the dipole antennae with a set on the captain's side by using a pair of splitter pigtails. After reading up a bit, it appears I should ensure that the cable lengths and types match to avoid signals destructively combining, and maybe cable length should be related to the frequency being received.
But I don't know how much that matters in 2, 1, and 1/2m cable.

Any advice from anyone who understands this wigglyamp stuff would be appreciated.
« Last Edit: April 01, 2021, 08:42:09 am by neilmurg »

Ian Melville

Re: Twin internal dipole aerials (total 4) with SMA Y pigtails
« Reply #1 on: March 31, 2021, 11:31:23 pm »
Good luck with that. Since you are transmitting into those dipoles you had better get the matching spot on, or it will do more harm than good.

Admin

Re: Twin internal dipole aerials (total 4) with SMA Y pigtails
« Reply #2 on: April 01, 2021, 07:53:28 am »
Hi Neil

I am no RF expert but I think antenna diversity is very difficult to get right in a passive implementation
I think most implementations use an active solution where the diversity is alternated between the two antenna

Lots of RF experts available for comment...
Thx
Lee

neilmurg

Re: Twin internal dipole aerials (total 4) with SMA Y pigtails
« Reply #3 on: April 01, 2021, 08:21:50 am »
thanks both. What reading I've done tells me there's a lot too it, more than I can bridge with some book learnin'. There's a antenna guy at our field that I am consulting.
Current results, vs the external Mode S look great looking forward. I guess that's partly an artifice of the system needing to MLAT my mode S
« Last Edit: April 01, 2021, 08:40:05 am by neilmurg »

exfirepro

Re: Twin internal dipole aerials (total 4) with SMA Y pigtails
« Reply #4 on: April 01, 2021, 10:03:16 am »
Hi Neil / All,

I certainly don’t consider myself an ‘expert’ (compared to those who do this sort of thing for a living), but I do have a fair bit of experience of using multiple stacked antenna arrays over the years in other (non-aviation) scenarios.

It is certainly possible to combine antennas using a passive matching stub. I have one here (home made but not by me) for combining two large ‘Yagi’ arrays for the 144MHz - 146MHz Amateur Band, and have also used similar units to run four combined antennas with pretty high power transceivers in both the above and the 432 - 433MHz band in the past. Unfortunately, I know of no-one making similar units (commercially or otherwise) for the frequencies we are operating on, though I have no doubt they could be produced if there was a big enough market. I will see what I can find out.

The significant issue with antennas is that to achieve effective reception or transmission of radio signals, the antenna(s) / antenna system needs to be as closely ‘matched’ as possible in terms of ‘impedance’ to that expected by the transmitter/receiver (normally 50 or 75 Ohms depending on the nature of the device - most avionics are 50 Ohm nominal). That said, there isn’t as much of a perceived  problem with received signals (viz. in our case the 1090 side), though even here it is always best to match the antenna system as closely as possible to what the receiver is expecting in order to achieve the best results.

Antenna matching is, however, far more significant in the case of radio transmission, where it is pretty much essential to ensure that the antenna system provides as close a match as possible to the impedance which the transmitter is expecting to see. Failure to achieve this (within certain limits) will result in part of the transmitted signal being reflected back into the transmitter, rather than being transmitted from the antenna(s). At best, this leads to reduction in range and effectiveness of the transmission. At worst (though less significant with low-power devices like PAW) the reflected signal can seriously damage or even destroy the output stages of the transmitter.

I have a few ideas in mind, but can’t try them properly until I get a chance to get back to the plane (we are still in ‘Stay at Home’ Lockdown up here). I will let you know how I get on.

In the meantime any help or comments from the real ‘experts’ will be appreciated.

Best Regards

Peter

neilmurg

Re: Twin internal dipole aerials (total 4) with SMA Y pigtails
« Reply #5 on: April 04, 2021, 11:51:21 pm »
That's great feedback, thanks guys.
Here's where I am, and what I've heard, as a ingenue
2 Antennae is(are) difficult, esp. if they transmit. There are cable run issues, matching frequency, and (probably, I assume?) the distance between the antenna.
But difficult isn't impossible.
I would appreciate if someone who knows what they're talking about, could crunch the numbers on cable length, for RG-48 (50 ohm), and also distance between antenna, for 869.525.
That would at least put me in a good position to test against VECTOR and gloriously fail, or, who knows? add something useful to what's known for the benefit of everyone...

I note that SE are again stealing good ideas and doing what they have previously said is unsafe.... plus ça change....

Ian Melville

Re: Twin internal dipole aerials (total 4) with SMA Y pigtails
« Reply #6 on: April 05, 2021, 11:01:27 am »
I did antenna and installation design theory when I was an apprentice. Those were the days when the transistor was in full swing and the IC was just creeping into use. Our course still concentrated on valves and transistors as it would be a while before Silicon could handle the transmitter power.
I am well aware of the issues, but not of a practical way to overcome them. Understanding the mathematics is one thing, but to make a practical solution you will need the kit to test the outcome. I can test VHF systems, but not UHF. I did start to look into a DIY solution and have some bits at home. 

Easiest solution would be a Rossettax2  :)

trident

Re: Twin internal dipole aerials (total 4) with SMA Y pigtails
« Reply #7 on: May 31, 2021, 11:31:09 pm »
Hi

I just came across your thread and it seems like you could achieve better results with just simple colinear-type aerials, i.e. those listed as with gain; 3db, 5db etc. These effectively "stack" aerials on top of each other and increase gain in the horizontal plane whilst reducing the gain vertically. This will have the effect of increasing reception range towards the horizon but reducing it above and below the aircraft. Your comments beg the question how far do you need to see, bearing in mind 5,000 ft should be more than enough above and below? Similarly, how distant do you need to see horizontally? It's always a compromise and a trade-off. Also, be aware the P3i is limited to 0.5W ERP (As far as I can remember), so you shouldn't be adding "gainy" aerials to this part of the PAW system.

As for joining aerials together, to do it properly and achieve the desired radiation pattern, you need aerials that are mounted at specific distances apart, known good aerial VSWR's that are fairly closely matched and phasing harnesses or power dividers/combiners made for the frequencies in question. With the aerials side-by-side and vertically polarised, you will achieve gain at 90 degrees to the plane of the aerials, i.e. one either side of the cockpit gives increased range in front and behind PROVIDED they are the requires spacing of (As far as I can remember) multiples of 1/2 wavelengths apart.

As Peter says, just joining 2 or more aerials together may appear to work on receive but it is not optimal because of the incorrect impedance and phasing, whilst transmitting into such a setup can quickly destroy the transmitter output device. Minicircuits make various splitter/combiners and a typical one that would do the job is: https://www.minicircuits.com/WebStore/dashboard.html?model=ZN2PD-183W-S%2B but it's not cheap! I use a similar 4-port one for a stacked and bayed 4 element array at 1296MHz for Amateur TV, but it was bought at a radio rally as surplus for £2.

Maybe consider mounting the aerials as far forward and in the centre of the windscreen as possible with a clear view forward. After all, that's where the biggest threat would be. Targets behind are less of a threat whilst those to the sides, above and below are medium risk.

Frank.

Ian Melville

Re: Twin internal dipole aerials (total 4) with SMA Y pigtails
« Reply #8 on: June 01, 2021, 07:32:20 am »
Frank, I think you are missing the reason why Neil is looking at 2 antenna. It is not because of signal strength, but to overcome airframe masking of the targets. Colinear would not resolve that.

exfirepro

Re: Twin internal dipole aerials (total 4) with SMA Y pigtails
« Reply #9 on: June 01, 2021, 09:10:29 am »
Hi Frank,

Thanks for your clear and helpful contribution. As per Ian’s response, however, the main issue we need to address is trying to improve coverage due to airframe obscuration, rather than achieving power gain for its own sake (which could of course as you imply make the P3i transmitter ‘illegal’ - unless any ‘gain’ is balanced by opposing ‘losses’ in the system).

I have a couple of splitter/combiners here, one being a second-hand ‘Minicircuits type’ - the other designed for WiFi networks, but both reported as suitable for our frequencies, but haven’t yet found the time to test the theory. The problem I foresee is that with around 3dB loss through the ‘splitter’ we theoretically need to bump up the antenna gain to compensate. Unfortunately whilst ‘doubling up’ the antennas should (in theory) reinstate at least part of the loss through the splitter this is likely to be ‘hit or miss’ and is in any case unlikely to result in a doubling of received signals (due to phase losses in signal path) - though it might give an indication of traffic from a previously obscured direction. On the other hand, adding ‘gain’ antennas to compensate for the introduced losses, could easily result in an increase in ERP. Unfortunately, whilst I can set up a ‘practical trial’ (when I can find the time), without the proper (very expensive) test kit, the results are likely to be little more than subjective opinion.

Best Regards

Peter

trident

Re: Twin internal dipole aerials (total 4) with SMA Y pigtails
« Reply #10 on: June 03, 2021, 11:08:39 pm »
Hi Peter and Ian,

Thanks for your clarifications.

Yes, maybe I missed the point a bit, but unfortunately as you say Peter, without proper test gear and time to experiment, it's going to be a rather hit and miss or is it "empirical" affair. Combining the signals even through a proper splitter/combiner will effectively lose 3db although that loss could be negated with aerials with 3db gain. Wherever the aerials are placed, unless they are optimally spaced, the polar pattern is going to be very hit and miss and not provide the solution. Just thinking about it, every such setup would be unique and dependent on the distance between the aerials which depends on the aircraft.

My suggestion was to put the aerial where it will have a clear view in the most required direction, i.e. directly in front. OK, there may be some obscuration from the airframe, but it would be even on both sides. A colinear would improve the gain in the horizontal plane, so the signal from the obscured direction would hopefully also be improved. With the warning range reduced in the software setup, maybe it would give near equal coverage all around.

In this situation and apart from mounting the aerial externally, I don't think there is a simple solution other than a true diversity setup, but that's another story.

Best regards,

Frank.