Actually, the 14 ft boom is near 'optimum' for 3L on 10M.
An 18 ft boom is near optimum for 3L on 15M.
A 24 ft boom is near optimum for 3L on 20M.
(Optimum gain with moderate F/B occurs
around .35 WL for 3L designs)
SO, what band do you want to optimize?
The CC A4S uses a separate 10M reflector and an 18 ft boom
which is somewhat better than the A3 on a 14 ft boom.
The Mosley CL33 used 3L on an 18 ft boom which is
over 0.5 WL on 10M. The curves for a 3L Yagi on a
0.5 WL boom in the W2PV Yagi Book show very LOW F/B.
There are MANY different designs for 24 ft booms with
additional elements for 10 / 15. You can line up reflectors
in band order but directors become a problem because a
single band lower freq director will look like an inefficient
reflector in front of the DE on the higher bands. This
effect can be broken up by traps or open sleeve arrangements.
Without an antenna range to determine your gain / pattern
and to do impedance matching for all three bands, it is best
to take the stock antenna and use it as designed. If you
change the spacing on your TH3, you will need to re-optimize
the tuning and matching for each band, a daunting task!
de Tom N4KG
On Wed, 07 Feb "Lonnie (NT6B)" <nt6b-vk7lm@dreamsoft.com> writes:
> Hi Tom and the group...It seems that the shortcoming of the TH-3 is
> somewhat obvious in that the boom length is too short. As the owner
> of a very nice TH-3, I would like to make it perform better than the
> stock version. It seems to me that lengthening the boom, advancing
the
> spacing to optimum would make the antenna "sing" and provide a much
wider
> bandwidth and overall better performance on all three bands. A good
friend of
> mine has a wide spaced 3 element and although a monobander, it seems
to
> be performing very very well.
>
> What would your suggestions be for lengthening the boom, spacing to
> make
> the TH-3 provide optimum performance. The antenna programs that I
> have
> will not incorporate trapped versions in their calculations. The
> TH-3 is
> currently down and I have plenty of boom material. As a owner of
> the
> Telrex 5 element 20 meter monobander on a 35 foot boom, the close
> spacing
> compared to the 5 element on a 46 foot boom version is very
> noticeable..Your suggestions on changing out the TH-3 are very much
> appreciated and I will make the appropriate changes and report back
> to the
> group on performance enhancements of the longer boom.
>
> 73's.....Lonnie (NT6B)
>
>
>
>
> At 05:30 AM 2/7/01 -0600, n4kg@juno.com wrote:
> >The TH3 has a 14 ft boom which is only 0.2 WL on 20M,
> >0.3 WL on 15M, and 0.4 WL on 10M. The maximum
> >gain for a FULL SIZE 3L20 on a 0.2 WL boom is close
> >to 7.0 dBi which is also the maximum gain for a 2L on
> >a 0.15 WL boom. Estimated trap losses are 0.5 dB.
> >
> >The main shortcoming of the TH3 is limited bandwidth.
> >It seems to do a good job in the portion of the band
> >for which it is tuned but my experience is that gain
> >(competitiveness) seems to drop off on the other end
> >of the band.
> >
> >The C3 is essentially 2 full size elements on each band.
> >On 10M the parasitic element is a DIRECTOR with
> >NO reflector. Personally, I prefer the C3S design
> >which utilizes 3 parasitic REFLECTORS on a shorter
> >boom or the C3E which adds a 10M reflector, making
> >it 3 elements on 10M for improved gain and F/B.
> >
> >Another option is a triband (or 5 band) 2L Quad.
> >Quads utilize full size wire elements which provide
> >good gain-bandwidth. Newer designs with heavy
> >duty spreaders seem to hold up under icing better
> >than older designs. There are at least 2 single feedline
> >Quads available on the market. I believe W4RNL has
> >details of a 5 band design on his WEB page that utilizes
> >separate feedlines which may be a better choice.
> >
> >de Tom N4KG
> >
> >On Tue, 6 Feb 2001 "Mike K9MI" <mdbrown@iquest.net> writes:
> > > This is a "what if" question, as I don't have
> > > the funds to do this right now anyway. But,
> > > maybe sometime in 2001...
> > >
> > > Right now, I'm using at TH3 tribander at 40 feet.
> > >
> > > Has anyone replaced a similar antenna (3 el trapped
> > > tribander w/ 14ft boom) with a Force 12 C3 ? I'm
> > > relatively certain, that yes the C3 is better. But
> > > how much better ?
> > >
> > > Mike, K9MI
> > >
> >
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