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Europa-List:Transponders

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nicolaprice(at)tiscali.co
Guest

Posted: Sun Jul 15, 2007 1:21 am Post subject: Europa-List:Transponders

Here's a teaser to get us away from the trauma of  mod 73,

 Where's the best place to site the transponder  antenna on a classic monowheel.

 Thanking you all,

  

 John Price.

 [quote]   ---

carl(at)flyers.freeserve.
Guest

Posted: Sun Jul 15, 2007 1:44 am Post subject: Europa-List:Transponders

Ours is mounted on the underside of the fuselage  about 12" forward of the tailspring attachment bolt.

  

 It is a small sharksfin design from Spruce and we  stuck a circular groundplane (made from bacofoil) to the inside of the  fuselage.

  

 The only downside is the length of the coax cable  to the transonder unit itself. Anything over 6 feet requires a beefed up coax  cable (cant remember the spec). If you buy it from specialist suppliers it is  darned expensive (and about 10mm in diameter). I seem to recall we found  something similar at much less cost from Tandy- bit it was 10 years  ago.

  

 Alternatively save some money and shorten the  distance to the aerial (ie: move it closer to the cockpit) - unless you are  planning to have kids !!!!!

  

 Remember that unlike a comms radio a  transponder transmits continuously so the exposure to radiation is  greater.

  

 Carl Pattinson

 G-LABS

  

 [quote]   ---

nwcmc(at)tiscali.co.uk
Guest

Posted: Sun Jul 15, 2007 2:03 am Post subject: Europa-List:Transponders

I would suggest as low as possible in the rear fuselage but away from metal parts such as the flap crosstube. I have mine on a bracket on the join of the fuselage halves on the starboard side. In most cases it is OK but some ATC units have problems receiving a signal when I am flying towards them (particularly if they are in the 11 o’clock position) which infers screening from the engine. I intend moving it at the next opportunity.  

   

Nigel Charles  

   

--

davidjoyce(at)doctors.org
Guest

Posted: Sun Jul 15, 2007 2:48 am Post subject: Europa-List:Transponders

John, I couldn't say whether it's the best, but an easy, simple and

effective place to mount it is at the bottom of the side of the plywood

tower that encloses the tailplane mass balance. Mine is a Bob Archer SA-005

antenna and has worked without any apparent problem for 5 yrs. Regards,

David Joyce, G-XSDJ

---

europa flugzeug fabrik

Joined: 13 Feb 2006
Posts: 65
Location: North Coast, USA

Posted: Sun Jul 15, 2007 5:58 am Post subject: Re: Europa-List:Transponders

 	  carl(at)flyers.freeserve. wrote: 	
		  ...(ie: move it closer to the cockpit) - unless you are  planning to have kids !!!!!

  

Remember that unlike a comms radio a  transponder transmits continuously so the exposure to radiation is  greater. 	


No, it does not transmit continuously, but only bursts of a few milliseconds (at partial duty cycle) only when our reply lamp blinks in response to an ATC radar sweep.  If we do the math on RF exposure, we absorb more RF energy from a cell phone than a transponder antenna a few feet away.



Fred F.

carl(at)flyers.freeserve.
Guest

Posted: Sun Jul 15, 2007 11:03 am Post subject: Europa-List:Transponders

I was always given to believe that RF from transponders was fairly potent 

stuff but im no expert and I dont know how it compares to mobile phones or 

comms radios.



I guess it depends on your definition of continuously but for my money a few 

milliseconds of transmission every 2 seconds adds up to a significant amount 

of RF energy. Its not continuous but must add up to quite a bit. Comms 

transmission on the other hand would amount to a couple of minutes per hour 

for most of us.



But if you say transponder antennas are relatively safe then I guess you 

know what you are talking about.  Fortunately no amount of RF is going to 

affect me in the parenting department - I had the snip 30 years ago ! - and 

the grand children are someone elses responsibility.



Carl Pattinson

G-LABS



---

houlihan(at)blueyonder.co
Guest

Posted: Sun Jul 15, 2007 12:55 pm Post subject: Europa-List:Transponders

Hi Carl



Believe Fred he is completely correct on this issue.



I recall doing a training course on the then new transistorised IFF coder

whilst in the RAF a considerable number of years ago. From memory the

individual pulses for mode A and Ident ( and I assume for mode C ) are each

less than a microsecond long and a transmission of a peak power of about 700

Watts ( as in my transponder ) will give a mean power of just a couple of

watts.

I will let you do the sums about all this in your own time  but your comment

that that it adds up to a significant sum is in my view wrong and misleading

europa flugzeug fabrik

Joined: 13 Feb 2006
Posts: 65
Location: North Coast, USA

Posted: Sun Jul 15, 2007 5:29 pm Post subject: Re: Europa-List:Transponders

RF energy harms body tissue by heating it, as in a microwave oven, which is continuous energy. However, my oven on the defrost setting zaps the food only periodically, like on for X seconds and off for Y seconds. Also, the  higher the frequency, the more the heating. VHF does a poor job, and 1 gHz (transponder) is better, and that's in the range of a cell phone also.   



Transponders are spec'd in peak power, and 250W is typical.  RF energy is transmitted only when we send a "1 bit" during a pulse train time duration of 21 microseconds, and how many pulse trains we send is based upon a whole bunch of variables.  Like how many 1-bits we send. How many pulse trains do we send when our reply lamp flashes? Not really relevant, as we shall see, but it could be a total of 2 milliseconds worth, as a guess.



So, 2 ms every 2000 ms cuts our avg 250W RF power to an average of only 250mW, roughly our cell phone's typical power (to save battery, they cut themselves back to minimum needed).  This comparison is not a good one, because we would need to multiply the transponder's 250mW times a 50% duty cycle for sending bits, then by sending typically only 40% of maximum bits, times another % because of the significant dead time between between each pulse train we send, and finally by another % to reflect the fact that a bottom-mounted antenna will not place our body in a strong part of the radiating pattern at all. And of course, unlike the cell phone, the distance to our antenna is at least feet away.  There's  a square in the formula for that, so double the distance and we attenuate the radiated power by 25%.



So, we're talking about an average  handful of milliwatts here rec'd by our body. Like trying to heat food in the microwave by turning it on for just 2 seconds each hour. In a week, it will still be room temp.



Fred F.

ami(at)mcfadyean.freeserv
Guest

Posted: Mon Jul 16, 2007 12:26 pm Post subject: Europa-List:Transponders

<<the fact that a bottom-mounted antenna will not place our body in a strong 

part of the radiating pattern>>



Fred,

Where is the "cone of silence" located on a typical transponder aerial?

Makes a case for sitting right on top of the aerial ( or axially above it, 

to be precise)!



Duncan Mcf



---

paul.mcallister

Joined: 09 Jan 2006
Posts: 177
Location: Waukesha, WI USA

Posted: Mon Jul 16, 2007 2:04 pm Post subject: Europa-List:Transponders

John,

  

 I had  a "spike" antenna located behind the baggage bay but I received continual  complains from ATC when I was flying towards them at altitudes of 4000' or  less.

  

 I  changed to a Bob Archer antenna and moved it down to the mass balance arm area  and I have never received a complaint since.

  

 Paul





    [quote][b]

grahamsingleton(at)btinte
Guest

Posted: Mon Jul 16, 2007 3:24 pm Post subject: Europa-List:Transponders

Paul

the radiation pattern of a dipole is like a figure 8 on its siude, 

(doughnut) is that right Fred? John? Quarter wave whip is more or less a 

sphere.

Graham



Paul McAllister wrote:

 	  Quote: 	
		   John,

  

 I had a "spike" antenna located behind the baggage bay but I received 

 continual complains from ATC when I was flying towards them at altitudes 

 of 4000' or less.

  

 I changed to a Bob Archer antenna and moved it down to the mass balance 

 arm area and I have never received a complaint since.

  

 Paul*

europa flugzeug fabrik

Joined: 13 Feb 2006
Posts: 65
Location: North Coast, USA

Posted: Mon Jul 16, 2007 5:10 pm Post subject: Re: Europa-List:Transponders

The radiating pattern on a 1/4-wave monopole (the common xponder stubbie thing, or whip-type comm antenna) is a half-donut.  Sliced laterally, so each half has a hole. 



See www.linxtechnologies.com/Documents/AN-00501.pdf



Above is theoretical in one section, and a practical matter there will be backlobes, as implied by one of their later diagrams as tested on an actual antenna.



Since I'm sure this radiation is harmless, a 1/4-wave antenna on the fuselage bottom at lowest point should be most effective.  On my tri-gear, I have it underneath the central tunnel, which has the advantage of keeping the coax short enough it makes little difference what type of coax it is.



Fred F.

europa flugzeug fabrik

Joined: 13 Feb 2006
Posts: 65
Location: North Coast, USA

Posted: Mon Jul 16, 2007 8:29 pm Post subject: Re: Europa-List:Transponders

Re the Archer dipole, there is nothing theoretical to suggest it should work any better.  Like any dipole, it should be more sensitive to nearby metal elements to degrade performance (including its own feedline, if not aligned properly).  The fanciful shape of the PC board antenna elements I imagine are quite intentional, but no generic text I'm aware of says that's the best way to do it. Only Mr. Archer knows for sure, and it may indeed optimize radiation pattern for xponder use.



However, the case with any antenna is that pure line-of-sight always works, 360-deg around the aircraft (looking through only fiberglass is OK). Also, how much actual xmit power your transponder has is important, because we can't compare user reports w/o knowing that.  In the U.S., where a periodic xponder test is required, the box will pass with considerably less power than the spec of any box currently sold (the King KT78/KT78A the only one on the weak side when new or working to spec). 



Fred F.

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