[TowerTalk] Rai Beam

[Michael Tope]

Hi Joe,

I think the new QST ad policy is to publish gain specifications if they are
backed
up by a computer model. The fact that RAI is claiming free space gain,
suggests
that these are modeled figures. The ad specifies that the antenna operates
under
"supergain" conditions. According to my copy of Kraus this is the same
principle
at work in the closed spaced W8JK array, which can in theory deliver up to
3.9
dBd (6.1 dBi) free space directivity for spacings on the order of 0.1
wavelengths
(J.D. Kraus, Antennas, 2nd ed, pg 519). Kraus goes on to mention that the
tight
coupling associated with the closed space W8JK array results in a very high
Q
antenna (narrow VSWR bandwidth) that is very susceptible to I^2*R losses by
virtue of its inherently low radiation resistance - a characteristic that is
inconsistent
with the broad VSWR bandwidth claimed by the RAI beam.

A quick look at the RAI website, confirmed my suspicion that the 8dbi gain
was
derived via NEC modeling. The website contains a short rather confusing
discussion
of how the NEC model was manipulated using "actual current and voltage
measurements" and a second trannsmission line (designed to overcome supposed
shortcomings in NEC transmission line models) to arrive at "satisfactory"
predictions
that matched "real world" measurements.  No mention is made as to how the
"real
world" measurements were made, and the supposed limitations in NEC
transmission
lines models that they mentioned, seem to have escaped the notice of W7EL
(the
transmission line limitations described in my EZNEC manual don't include any
mention
of one-way power flow).

All-in-all, the explanations given on the web site sound a little on the
fishy side. In the
same manner that manfucturers can fudge and manipulate measured data, it
seems that
the RAI folks may have figured out how to fudge and manipulate computer
modeling.
While I suspect the RAI is probably a good antenna probably that works about
as well
as any other 2 element yagi, claims that it can deliver 8dBi free space gain
seem dubious.
Perhaps the timing of the ad (April issue) is no coincidence.

73 de Mike, W4EF


----- Original Message -----
From: Joe Reisert <jreisert@jlc.net>
To: <towertalk@contesting.com>
Sent: Monday, April 10, 2000 4:14 PM
Subject: [TowerTalk] Rai Beam


>
> To Interested Antenna Buffs,
>
> I must have deleted the prior message from a happy guy in New Mexico who
> just got on 10 meters with a  2 element Raibeam. Raibeams gain claims have
> always been, well, rather high, so I just discount them out of hand and
> turn the page!
>
> Today, my curiosity got the best of me so I decided to look further. I
> found one of the latest Raibeam ads in the April 2000 QST on page 132.
It's
> a tri-bander with what looks like 3 separate 2-element interlaced Yagis. I
> almost flipped out for two reasons.
>
> 1. The  ARRL actually published the Raibeam's claimed gain. I've heard
ARRL
> has done it before but I have never seen any such ad. Was this a mistake?
> Was it slipped through the editorial department? Did it pass the new so
> called antenna test data and models they have imposed on all published
gain
> claims in QST?
>
> 2. If it is true gain, we better all dump our antenna software, get a
> Raibeam and take down those big beams we are now using. Greater than 8 dBi
> gain in "free space" on 2-element a tri-bander using only an eight foot
> (0.12 wavelength on 20-meters) boom is mighty impressive. Furthermore the
> pattern in the ad infers a F/B ratio of almost 30 dBs!
>
> Is this antenna patented? If so, what is the patent number so I can get a
> copy and be enlightened.
>
> I'm anxiously awaiting the new antenna gain theory!
>
> Joe, W1JR
>
>
>
>
>
>
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