[Lf] Mobile Antennas for Antenna Efficiency Msmts]

[Andre' Kesteloot]

"Ashlock,William" wrote:

> Hi Bob and All,
>
> Just a few words on a subject close to my heart. I've been experimenting on
> the Lowfer band for over 4 years but it wasn't until I designed a good
> portable receiver two years ago with a multi-range S-meter (call it a
> frequency selective voltmeter) that I made real progress in antenna design.
> I don't want to imply that antenna design is an empirical process but I
> found that it is absolutely necessary to follow up any
> theoretical/mathematical design change with an actual measurement that will
> give, to a high degree of accuracy, the result of this change.
>
> My experience has lead to the following conclusions about the needed
> receiving equipment:
>
> Receiver: The performance must be almost as good as the best Lowfer
> receivers. The bandwidth should be < 400Hz. The S/N at 1uv should be better
> than 20db (for signal checking under 20 miles this could be reduced to
> 10uv). The S-meter must be linear responding and must cover at least 3
> decades of signal strength beginning at a full scale reading for a 1uv (10uv
> for reduced range) signal. The relative signal strength measurement error
> must be < 5% of reading and the absolute accuracy error must be < 25% of
> reading. The receiver (and antenna amplifier) should have a good rejection
> to AM band signals and any other strong IM generating sources such as Loran
> C. Extraneous signals and spurs can lead to time-wasting false readings.
> Typical DC (direct conversion) receivers do not meet these requirements.
>
> Antenna: I recommend a magnetic mounted 1 meter active rather than a loop. A
> loop is definitely difficult to manage in many situations including being
> outside in foul weather, the usual holding in two hands: the receiver, the
> headphones, the antenna, and the usual snarl of wires running between these
> - not to mension the need to tune the loop. With the active setup you stay
> in your car with all the gear sitting on the seat next to you.
>
> One negative is that the active antenna requires at least 120ft of tree
> clearance for accurate readings but there are many spots, city and rural,
> that meet this requirement such as supermarket parking lots, cemeteries,
> school grounds, and (safely) off the side of interstate highways. Power
> lines on the side of most roads are a problem for both types of antennas.
> The calibration of a loop is usually not a simple matter. The calibration of
> an active antenna is relatively simple with a standard RF signal generator.
>
> Any comments/experiences appreciated.
>
> Bill WA
>
> -----Original Message-----
> From: Bob Roehrig [mailto:broehrig at admin.aurora.edu]
> Sent: Monday, August 28, 2000 9:21 AM
> To: Don Burns
> Cc: lowfer at qth.net
> Subject: Re: [Lowfer] Mobile Antennas for Antenna Efficiency Msmts
>
> On Sun, 27 Aug 2000, Don Burns wrote:
>
> > What kinds of antennas and receivers are you guys successfully using for
> > field strength measurements when checking antenna efficiency?
> > --
> > We used some very interesting measurement gear to do this job
> > but I'll be danged if I can recall who made it or what it was called.
>
> Our was a Nems-Clark unit - quite old, it had miniature tubes in it.
> The cover contained the loop antenna and it had a calibration osc in it,
> otherwise it was nothing more than a regular superhet with a fancy
> S-meter.
>
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