What’s this antenna?

[India Four Two]

I’m not proposing a new quiz. 😄

I stumbled on this photo of an RAAF Sunderland in Rose Bay.

What’s the function of the Yagi antenna on top of cockpit?

https://cimg1.ibsrv.net/gimg/pprune.org-vbulletin/1170x878/img_0967_092a69025e36a0acbb43f7344439dd7272b6ab1b.jpeg

[Quemerford]

Looks very similar to the ASV aerials carried on Barracuda etc.

[garyscott]

It’s a Sunderland Mk III, dubbed the Porcupine due to the extensive aerials. The one above the cockpit area is literally the tip of the iceberg! ASV Mk.II, Transmission Yagi above the cockpit, and receiver Yagi aerials beneath the wings. I think the spine antennae are the “Broadside” system?

[Compass Call]

Looks like my TV aerial :}
I'll get my coat!

[First_Principal]

Agree with garyscott, a fair writeup on the system may be found here (https://uboat.net/allies/technical/uk_radars.htm), from what I can guesstimate the antenna looks to be about the right size for the frequencies involved.

A quick double-duck search shows images of a few machines sporting similar antennas including Hudsons, Liberators and PBYs, along with the Sunderland.

FP.

[BEagle]

Looks like my TV aerial

Hardly surprising given that ASV Mk.II used 176 MHz and Band 3 TV used 174-240 MHz.

[India Four Two]

Thanks garyscott and First_Principal. I've been interested in radar for years, so I'm surprised I hadn't seen that antenna before.

There's a good Wikipedia article on ASV Mk II. I particularly like this extract:
Much of this [work] involved the development of new antenna systems, more advanced than the system on the Anson where a dipole was held outside the escape hatch and rotated by hand to hunt for signals.

https://en.wikipedia.org/wiki/ASV_Mark_II_radar

Looks like my TV aerial https://www.pprune.org/images/smilies/badteeth.gif

That's because they are both Yagis. :)
https://en.wikipedia.org/wiki/Yagi–Uda_antenna

Yagis come in all sizes. Here's one similar to the HF antenna on top of my company's office in Jakarta in the 1980s.
https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/1200x1186/lph1a_img_21169b421f726ba3dc0ccdae40d1feffc529c044.jpg

We used it to communicate via radio-telex with an oilfield in North Sumatra - a distance of about 1600 kilometres. Our radio man, Hal* was complaining to me one day that he could transmit telexes to the field with no problem, but receiving was very unreliable. Eventually, a drilling engineer arrived back from the field and said "Hal, I don't know much about radio, but shouldn't the narrow end of the antenna be pointing to Jakarta?"

* Required aviation content: Hal had been a Bf 110 radar operator. He told me that he met his wife at the funeral for a Lancaster crew that had been shot down. If you look up the Bf 110 on Wikipedia, you'll see festoons of Yagi aerials.

[radar101]

"Hal, I don't know much about radio, but shouldn't the narrow end of the antenna be pointing to Jakarta?"

​​​​​​​Yes, the backlobe wouldn't be very strong

[Asturias56]

Hey! Its Indonesia - at least they had it in the right plane.....................

[Quemerford]

Happy to help! :)

[EXDAC]

Yagis come in all sizes. Here's one similar to the HF antenna on top of my company's office in Jakarta in the 1980s..

That looks more like a log periodic antenna to me.

[First_Principal]

That looks more like a log periodic antenna to me.

Quite agree, would have allowed Hal to use a wide range of frequencies - but still needed the small end to the assumed location of the signal (assuming short path)! ;)

FP.

[India Four Two]

EXDAC and F_P,

I see that log periodic and Yagi are related, so I don't feel too bad in misidentifying the antenna. ;)

[First_Principal]

EXDAC and F_P,

I see that log periodic and Yagi are related, so I don't feel too bad in misidentifying the antenna. ;)

Oh yes, no criticism intended, but what it tells us is that Hal likely used multiple frequencies to converse with the remote site.

This is normal procedure in the HF aviation world where, as much as is possible, 24/7 long-range communication is desired. As propagation/site separation changes certain frequency bands become better or worse for communication and so you may find aircraft and ground stations change frequency to a more suitable one during flight, often designated 'primary' and 'secondary' frequencies. Interference from OTH radar and other cr*p can also influence a frequency choice.

As well as the transmitter and receiver combination needing to be capable of such wide frequency changes (let's say 8.8MHz/33m and 13.2MHz/22m) so does the antenna, particularly for the transmit energy to be efficiently radiated. Antenna suitability also assists with the receive signal, but perhaps to a lesser degree.

For most aircraft installations you need an omnidirectional antenna, which a yagi (or log-periodic) is not as these antennas will 'beam' in a particular direction - although not perfectly as @radar101 and others have alluded to here. Additionally a typical yagi is a relatively narrow-band antenna, frequency-wise, and so would not be suitable for typical 24/7 site-to-site phone communications where you need to change as propagation changes. You could add a second set of elements, but it starts to become complex with feeds etc, and what if you wanted to use a third frequency? So, for when a directional antenna is required this is where the log-periodic comes into its own, they can be designed to be quite wide-band, albeit at the expense of 'gain', but that's another subject.

Omnidirectional antennas can also suffer from the narrow frequency 'problem', particularly if you want good gain (there's a clear tradeoff), however they can also be constructed as a broadband array or single unit. A modern example of the latter which many will have seen on the ground is a 'discone', typically used on higher frequencies. Early aircraft used to just have a dangling wire that was wound out when in use, and the transmitter adjusted to load into said wire as best as able (although the wire would have had an ideal length). Said wire's radiation pattern wouldn't have been perfectly omnidirectional but it'd have been close enough for most purposes, and in extreme situations you could just turn the a/c. Ground stations have the luxury of space and fixing area so you could simply have multiple antennas and switch them accordingly, or use quasi-directional multi-band antennas, but again that's straying into another area...

Apologies for getting carried away, but I thought it could be of some interest.

FP.

[India Four Two]

No need for apologies, FP. That's a very interesting post.

Some questions for you.

1. Both stations were in roughly the same longitude and communications were only required during the day. Would there have been much need to change frequencies?

2. I don't remember if there was any voice traffic on Hal's link to North Sumatra, but most, if not all traffic was radio-telex. Can telex traffic tolerate higher noise levels than voice traffic?

3. When tha antenna was pointing the wrong way, were we receiving transmissions off the back of the array or was the signal going round the world?

[EXDAC]

HF propagation is fickle, changes throughout the day because of sun illumination of the ionosphere, and also changes long term with the sunspot cycle. The VOACAP predicion tool is useful but, in amateur radio, sometimes you just get lucky by being on the right band at the right time.

https://www.voacap.com/hf/

Log periodic antennas will have radiation off the back. How much is defined by the "front to back ratio" but I have no idea what that was for this antenna.

Back scatter can allow propagation on a path inside the skip distance and, to exploit that, the antenna is pointed away from the other station. Back scatter is not long path as the signal starts off in the wrong direction and then is scattered in the right direction..

Radio teletype (RTTY) will work with weaker signals (lower signal to noise ratio) than SSB voice.

[golfbananajam]

I’m not proposing a new quiz. 😄

I stumbled on this photo of an RAAF Sunderland in Rose Bay.

What’s the function of the Yagi antenna on top of cockpit?

https://cimg1.ibsrv.net/gimg/pprune.org-vbulletin/1170x878/img_0967_092a69025e36a0acbb43f7344439dd7272b6ab1b.jpeg
TV reception ;-)