In a message dated 2/16/2004 7:32:03 PM Eastern Standard Time,
n3eu@comcast.net writes:
> If using RG-400 coax or equivalent as one should, 9 ft. will be about 8%
> lossier than 6 feet. The danger of human exposure to transponder replies is
an
> old wives tale. If a 250W xpondr is replying at the extraordinary rate of 1
> per second, a distance of 1 inch from the antenna is safe according to our
> FCC's RF safety guidelines (and online calculator).
>
> Regards,
> Fred F.
>
Hi All,
I've been spending most of my time working on my bird, and less reading the
list lately, so I'm running about 2 weeks behind on reading the posts on the
forum. This RF stuff is right down my alley (I do cellular, PCS and microwave RF
testing for a living) so here goes.
Roughly 96% of the radiated energy is dissipated in the first two wavelengths
distance from the antenna. Couple that with the fact that your body is about
72% water which results in "skin effect", the phenomena that makes the radio
waves pass around your body, not through it. Therefore, consider a few inches a
safe distance from transponder antennae. Skin effect is the same reason why
cell phones DO NOT cause cancer. Remember, it takes 600-1000 watts of highly
concentrated microwave RF energy to pop your popcorn or thaw out that chicken
for dinner, so 200 watts of highly dispersed RF isn't going to do diddly to your
DNA. What you do not want to do is spend a lot of time looking at the antenna
up close when it is transmitting. RF can damage your retinas.
In regards to transponder antennae and placement, I'm using the Advanced
Aircraft Electronics transponder antenna mounted on the right side of the
fuselage
(vertically polarized) about 14" in front of the rear bulkhead.
I've tested this arrangement using an Anritsu-Wiltron 251S Sweep Generator
with RG-58C/U and RG-400 at 1090Mhz and the cable loss difference between the
two was negligible at this frequency. Keep in mind this is a serious piece of
test gear, not a $50 SWR meter from Radio Shack, so I'm confident and pleased
with the results. The aggregate SWR for this antenna, with good quality RG 58C/U
is running about 1.3 and the return loss for the coax, using a precision 50
ohm load at the antenna end, is in the dirt too. There is also no apparent
interference problem from any of the metalic objects (pitch push rod, counter
balance, etc) in the vacinity of the antenna. I'm installing the Garmin 320A
transponder. It puts out something on the order of 250 watts when transmitting.
I've not done any field strength testing, but with the numbers I'm seeing in my
testing I think ATC will "see" me just fine.
I also have an MFJ-259B for COM testing, kind of Mickey Mouse compared to the
Wiltron, but it works. I'm installing an Advanced Aircraft Electronics COM
antenna mounted on the tailpost, vertically polarized. Using RG-58CU and Amp
connectors I have an average aggregate SWR of 1.4 across the COM band. The
return
loss for the coax is kind of high at 4.8 dB, so I might be looking at some
other coax for the COM feed line.
The bottom line here is this, if you want good performance from your radios
use good quality coax and good quality connectors and install them properly.
Use connectors that have SOLDERED center pins, not crimp on ones. The crimp on
center pins might work fine now, but they will corrode over time and degrade
your transmit and receive performance. The connector should also crimp tightly
onto the shield. Use the right tool for the job, not a pair of pliars! Don't
use clamp on type connectors unless you want trouble down the road, either.
Scratch Radio Shack off your list of vendors for either of these parts.
BTW, if anybody wants their antenna system(s) checked, I'll have all my test
gear available at our fly-in in June. I can test anything from 25 Mhz to 3.3
Ghz.
Regards,
John Lawton
Dunlap, TN
A-245 (baggage bay bonded in yesterday, now I'm trying to figure out how to
spray Zolatone)
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