Re: External Antenna on Commercial Flights

From: Jonas Lonnroth (y#y#z#_at_griffel.se)
Date: 08/14/04 

Date: Sat, 14 Aug 2004 23:02:08 +0200

"Robertwgross" <robertwgross@cs.com> wrote in message
news:20040814140556.15298.00001635@mb-m04.news.cs.com...
> I repeat an old GPS story here.
>
> I was boarding a short flight from Los Angeles to San Jose, CA. In
advance, I
> had checked the web site of the airline, and it indicated that GPS
receivers
> were allowed (at that time) once above 10,000 feet. So, I had my GPS
receiver
> along. As I entered the cabin door, I stopped by the open cockpit door and
> thrust my GPS receiver forward, asking "Is it OK for me to use this once
above
> 10,000 feet?"
>
> The male pilot turned around, saw it, and responded "No. No GPS allowed at
> all." The female co-pilot heard that short conversation and took note.
>
> Dejected, I took my seat and kept my receiver stowed away. After takeoff,
we
> reached 10,000 feet. Shortly thereafter, the co-pilot came back into the
cabin
> and spoke with me. She said that she looked it up in their current
operations
> manual, and the receiver was allowed after all, so I was free to use it.
>
> The pilot was probably just having a bad day.
>
> I hooked up my receiver with an external antenna, and placed the antenna
up
> under the shade. Then I covered it with a pillow, so as not to attract
> attention.
>
> ---Bob Gross---

When talking about external antennas here, it would be understood as one
with a linear amplifier, often with up to 30 dB amplification. The crew
can't know whether your GPS actually shuts off it's internal antenna when
the cable to the external antenna is connected. You hold the GPS receiver
closer "than normal" to the external antenna while in a passenger seat. So
is the fear that a fedback loop could occur totally wrong? As a radio
amateur, I'd be worried about the risk of oscillation in unknown GPS devices
with external antennas connected. Anybody who has built high gain RF
amplifiers have had to worry about oscillationg feed back loops. AFAIK, 30
dB is high gain. Besides, there could be something wrong with the cable
shield also, to facilitate RF feedback.

I've noticed that makers of reradiating antennas warn users to separate the
input and output antenna elements well to avoid oscillation. These
amplifiers are nothing but the linear RF amplfiers contained in amplified
external antennas like the Gilsson.

I wouldn't use an external antenna on a GPS in an airplane without having
tested that feedback does not occur. But most people that complain that they
are not allowed to use the antennas have no idea about RF amplifiers and
feedback loops and it seems that they couldn't care less. The only thing
they think about is their right to use equipment that they lack a technical
understanding of.

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