"The versatility is well worth a small loss in directivity to me. "
This is a reasonable trade off, i.e. versatility vs. performance.
I assume you do understand that antenna gain comes from directivity
and therefore, a loss of directivity represents a loss of GAIN.
The W2PV Yagi Antenna Book has a curve showing gain vs.
boom length for 3, 4, 5, 6 and 7 element Yagi's. The curves
are fairly close to each other where there is sufficient coupling
between elements. The gain curve is NOT linear, i.e. it rises
fairly rapidly from .15 to .35 WL and again from .7 to .9 WL
>From .35 to .7 WL, the slope of the gain curve is less.
Gain vs. Boom Length is roughly as follows:
WL EL Gain (dBi)
.15 2 6-7
.25 3 6.5-7.5
.35 3,4 8.0 - 8.5
.45 4,5
.55 4,5 slow rise in gain vs boom length in this region
.65 4,5
.75 4,5,6 10.5
At BEST, multiband Yagi's approach the gain of short boom
3L Yagi's. Many have even less gain, more closely resembling
the gain of 2L full size Yagi's. This also includes typical LP
antennas. If you want maximum gain, you need to go to
monoband antennas (or stacks), preferably on separate towers.
There is no free lunch.
Tom N4KG
On Fri, 22 Jun 2001 "Thor Hallen" <thorh@worldnet.att.net> writes:
> The log periodic design in the ARRL Antenna Handbook is not one that
> I would
> recommend. I have made comparisons with another local ham working
> common DX
> stations simultaneously and concluded that the Tennadyne T6 with a
> 12 foot
> boom performs at least as well as the C3SS that it replaced with a
> 12 foot
> boom on 20, 15 and 10 meters and is superior on 12 and 17 meters. It
> is also
> nice to be able to operate on the MARS net frequencies outside the
> amateur
> bands and have good SWL reception for things like WWV. As a bonus,
> the T6
> also performs quite well on 6 meters. The versatility is well worth
> a small
> loss in directivity to me. Also, I have CC&R antenna restrictions
> and was
> pleased that people have commenting that "the new antenna is much
> more
> attractive than the old one".
>
> Thor Hallen K5AGE
>
> -----Original Message-----
> From: owner-towertalk@contesting.com
> [mailto:owner-towertalk@contesting.com]On Behalf Of Phil Duff
> Sent: Friday, June 22, 2001 4:09 PM
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] log periodic antennas
>
>
> I've been curious about log periodics myself.
>
> The ARRL Antenna Handbook 17th Ed. has a 5 band (13-30mhz) design on
> a
> 26 ft boom with 12 elements.
>
> The article text on pg 10-10 states in part: "...this is a large
> array....
> with the same gain
> one would expect with a full size two-element Yagi array:"
>
> I acknowledge the benefits of the LP including bandwidth, feed
> impedance,
> etc.
> but for a 26 ft boom and 12 elements I'd hoped for more than
> 2-element
> Yagi performance.
>
> 73 Phil NA4M
>
>
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