On 6/20/2016 12:04 PM, Patrick Thomas wrote:
At one point I considered trying to pack a couple common bands onto one yagi
boom, as has been attempted by the MFJ 1760 and a couple trapless HF antenna
designs. Not sure if anyone has perfected that approach, but I'd be curious to
hear of any promising experiments.
Patrick,
I don't have any experience with the MFJ-1760, but I own two similar
Cushcraft antennas: the A270-10S, an interlaced dual band yagi with 5L
on 2M and 5L on 70cm, and also the A270-06S, a similar antenna but with
3L on 2M and 3L on 70cm.
At first glance, these antennas (as well as the MFJ-1760) appear
attractive for rover operations; offering two bands (2M & 70cm) on a
single boom with a common feedline. The common feedline is convenient
when used with most of the modern tri-band (50/144/432) VHF/UHF radios
where 2M/70cm share the same antenna connection.
However, there are some significant compromises with these antennas
which impair the performance on 70 cm. Interlacing the 70 cm elements
in between the 2M elements has a detrimental effect on 70 cm, because
the 2M elements act like 3/2 wave elements on 70 cm and thus disrupt the
pattern of the 70 cm yagi. I discovered this when I created an EZNEC
model of the A270-10S and found the gain lower on 70 cm than 2M. If the
2M elements are removed, the gain of the 70 cm yagi INCREASES BY 2 DB,
WITH NO OTHER CHANGES. I believe the MFJ-1760 would suffer the same
performance impairment.
In his article about the tri-band (144/222/432) quad for roving
(http://www.n6nb.com/vhfquad.htm) which was developed on an antenna
range, Wayne/N6NB noticed the same thing, so apparently this interaction
between 144 & 432 elements is present with quads as well as yagis.
The other compromise with these duo-band Cushcraft beams is the feed
system. Each yagi section (2M & 70 cm) has a gamma match on the driven
element (which is OK), fed with a short piece of coax, then both coaxes
are simply joined together with a UHF "T" connector and then to the
single, common feedline. Due to the gamma matches, this is difficult to
model on the computer to determine any detrimental effects. While the
SWR of the yagis appears to be good when connected together, the SWR is
not so good when connected separately, which shows that on both bands,
the "other" yagi section is affecting the SWR. I can't comment whether
this is a problem with the MFJ-1760, as I don't know how the yagis are fed.
Both of these problems can be fixed. The poor performance on 70 cm can
be alleviated by extending the boom of the antenna on the front end and
mounting the 70 cm elements to be completly in front of the 2M
elements. I did this with my A270-06S (3L/3L) and used it on a rove
with what I considered good results. N6NB did the same thing with his
tri-band quad, pushing the 432 MHz element forward ahead of the 144 and
222 MHz elements and saw the gain restored on the antenna range.
Fixing the SWR interactions from the shared feedpoint could be remedied
by using a duplexer in place of the "T" connector. I have not yet tried
this with my antennas, but there should be no reason why it wouldn't
work. Such duplexers are readily available from Comet and others, or
WA5VJB describes an inexpensive home-made version (which he calls a
"band splitter") in his description of Cheap Yagis for Low Earth Orbit
(LEO) satellites. Check out
http://www.wa5vjb.com/references/Cheap%20Antennas-LEOs.pdf. Kent must
have discovered the same interaction problem in developing the dual band
(2M/70cm) cheap yagis for satellite work as his designs have the 432
portion in front of the 2M portion.
73, Dave/K8CC
_______________________________________________
VHFcontesting mailing list
VHFcontesting@contesting.com
http://lists.contesting.com/mailman/listinfo/vhfcontesting
|