[VHFcontesting] Log periodics

[David Pruett]

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