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A cheap way to get 8.33 - an "approved" handheld radio

My tests with my 2002 Icom A22 or A23 (? – the long one, which does the VOR radial) was at least 10nm and probably 20.

It was used with a dedicated rooftop antenna. You can’t do a “tee” anyway because you can’t feed RF up the back end of another radio. This has to be wired as a loop in the RF cable, which you unplug and then you plug the radio into it. To avoid plugging the handheld into the wrong socket (which would make it drive the other radio’s output) you should use different type connectors. I used a BNC (50 ohm) for the rooftop antenna, and an SMA for the existing radio. It all needs to be done with high grade cable (RG400 if possible – £8/metre and a bit stiff, which is why most installers don’t use it) because VHF radiates like hell, and most radios have significant 11th and 13th harmonics (which end up on 1575MHz – GPS!). I have some RG400 and would be happy to send short pieces to a few people

Administrator
Shoreham EGKA, United Kingdom

I’ve used an old Icom A21 as the only communication radio in one of my planes for many years. I use an external antenna and have never noticed much (if any) difference between it and installed radios, as used in my other plane. I’m based within Class D airspace so it gets used every flight. The aircraft has no generator so I run it on cheap AA batteries these days – I’ve found them easier to deal with than the rechargeable type. I’ve never lost any of the frequencies in memory due to battery issues.

Approval etc is not an issue for US operations. If its good enough for the FCC approval to sell it, presumably its good enough for any purpose including use in my plane.

Last Edited by Silvaire at 28 Dec 15:03

I’ve got a vertex 220 and its brill. It comes with an adaptor that allows me to plug my David Clark in. With the rubber duck airel it has a ten mile range at 2000 feet. I haven’t tried it with an external airel.

It lasts at least 24 hours of the standard battery pack and you can get a larger capacity pack off eBay for about a tenner. I power it using a fag charger which I brought off eBay for 5 quid.

Although mine only receives on 8.33 they have now released a fully comparable 8.33 version.

Perhaps what dissapoints me the most is that the sound quality is as good if not better than every certified radio I have ever used and cost about a 20th of the price.

It reaaly makes me wonder what benefit certification offers.

makes me wonder what benefit certification offers.

It’s a question many ask.

“Certification” enables an aircraft to fly all over the world without hindrance, non-commercially, so long as the pilot’s papers were issued by the State of Registry of the aircraft.

I am certain that if it wasn’t for ICAO, private flying would not exist outside the countries that have a long history of private flying i.e. USA, UK, France, Germany, Australia, NZ and a few others. Everywhere else it would be banned because such levels of personal freedom simply cannot be permitted by any half respectable dictator.

Flying is already difficult in many places (overflight permits are needed) but it doesn’t bother most of us because we rarely go there (much of Africa for example). But you can fly there if you go through the process.

The certification process requires items to conform to some requirements e.g. ambient temp, shock testing, etc. It is flawed in many ways, and this is not made easier by loads of people making money out of the system itself and out of making it as hard as possible. That’s the same problem in every other regulatory area e.g. ROHS where manufacturers of lead-free solder are pushing like crazy to ban leaded solders because unleaded solders are 5x to 10x the price (well, ones that actually work really well).

The system massively favours highly resourced players – look at how Garmin has almost taken over the world.

Whether this price is worth paying in aviation depends on how much flying freedom you want.

Last Edited by Peter at 28 Dec 18:35
Administrator
Shoreham EGKA, United Kingdom

I used a BNC (50 ohm) for the rooftop antenna, and an SMA for the existing radio.

This is the best solution. It’s also simple to make a loop on a side of the instrument panel. When you have a female BNC bulkhead connector on the instrument panel which attaches to your VHF COM. The coax cable on the antenna would have a male BNC connector, which would normally be connected to your radio. In emergency you could disconnect this male BNC and plug it in to your radio. One must be sure not to transmit with the fixed aircraft radio with the antenna disconnected.

An extra antenna would be better but might require additional installation approvals depending on the regulations applicable to that particulair aircraft.

It all needs to be done with high grade cable (RG400 if possible – £8/metre and a bit stiff, which is why most installers don’t use it) because VHF radiates like hell, and most radios have significant 11th and 13th harmonics (which end up on 1575MHz – GPS!).

I don’t agree on that. As this are short pieces or wire, and the frequency quite low, it doesn’t make much sense to use RG-400 instead of the RG-58. It won’t harm, but you won’t be able to notice the difference anyway, it will be even quite hard to measure the difference reliable on such small differences in attenuation. The advantages of RG-58 will also be that it is more flexible, and better suited for a connection which is going to be attached to a handheld radio.

Also for the GPS interference the coax cable wouldn’t make much of a difference, actually the RG-58 would be better then RG-400, as RG-58 attenuates this higher frequencies more than RG-400 does. The harmonics are transmitted anyway at the antenna, so if the handheld radio hasn’t good enough filtering, a notch filter could be used.

For high frequencies, suchs a GPS, transponder and DME in the GHz range, it does make sense to use good quality RG-400 cable, altough often, RG-58 is still acceptable to the manufacturer and from a maximum allowable attenuation point of view.

JP-Avionics
EHMZ

You may be right sometimes, Jesse, but I can prove the contrary.

See this.

I proved that the VHF harmonics (coming out of the KX155A/165A radios) were coming out of the short piece of coax between the radios’ rear RF connectors and the centre stack bulkhead connectors (BNCs). Those cables are about 0.3m long, and inaccesible short of extracting the entire centre stack. Socata did that with RG58 or similar.

Also I would not suggest using RG400 for the Icom-panel cable, which as you say needs to be nice and flexible. Also I am not saying the attenuation is that important between RG400 and RG58. But the RG400 is massively better shielded.

If one has harmonics coming out of the VHF antennae, and nowhere else, that is solvable either with the notch filters I show in the above article, or with the new rod antennae which combine a GPS+VHF antenna and which have a tuned cavity notch filter at the base, like the one used here.

The problem with harmonics leaking out of VHF RF is that once the stuff gets out, you are stuffed. Your GPS won’t work properly. It will be marginal on the fix quality but the pilot will probably never find out why it happens only sometimes – it is very VHF frequency dependent. It can also screw up the DME.

Administrator
Shoreham EGKA, United Kingdom

Interesting Peter!

I wonder what cable they used for the GPS?

There is some differences between different brands RG-58, some claim to be RG-58 but have a very open (non covering) shielding. I had this problems on several Robin aircraft with KX-125 and KX-155 and GPS systems, always was able to solve with the notch filter.

Also wonder if they did the interference test when this equipment was installed in your aircraft, as you where also questioning. It seems not. Or there must be something wrong with connectors.

Note, all Garmin equipment I know have enough filtering to eliminate GPS interference.

JP-Avionics
EHMZ

There is “fake” RG400 too. There is also a type of RG400, with a very similar name, designed for airliners where there is so much cable that a coax which is 1/2 the weight can save say 200kg (=1 modern passenger ).

I don’t think Socata did those tests. I don’t think it is bad connectors, because both radios do it.

I even opened up a KX155A radio to see if one could fit a notch filter inside it (the best place to put it) but there was no room inside!

Do Garmin actually put notch filters in there? I doubt it can be eliminated by design alone. Well obviously not totally – that’s impossible. Collins do / used to sell special “GPS compatible” VHF radios, for the King Air type of market.

If/when I get the centre stack re-done I will use semi-rigid coax inside I will definitely have to free-issue it to the installer though.

Administrator
Shoreham EGKA, United Kingdom

I do not know which kind of filtering they use, the manuals only state that no notch filters are neccessary. I agree that it would be hard to get the required harmonics attuenation with regular L/C filters. A notch filter is very efficient and gives a lot of attenuation.

Can you recall which coax cable was used for the GPS installation? (RG-58 was quite common their as well, and less suitable for that frequency range).

JP-Avionics
EHMZ

I have no idea what cable Socata actually used but it looks like RG58. However some of the French stuff has funny colours. For example their normal wiring (twin with a shield etc) is blue. It isn’t the normal Raychem 55 or whatever.

My GPS antennae were never rewired – at least not recently. None of them had been relocated. One day, I will get it all re-done with RG58. The only chance I get for this work is during the Annual, when I rent a hangar.

Administrator
Shoreham EGKA, United Kingdom
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