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My actual
construction method would not suit anyone else so will not be shown
here in any detail ( photos above excepted) BUT the final test results
were interesting.
The spacing
of the turns (and hence actual inductance value) of the base loading
coil was not particularly critical, no real change in SWR was noted
as it was compressed and expanded a bit to change the actual inductance
value..
After this test, a large diameter heatshrink was applied over the
coil assembly to give mechanical stability and thus a fixed value
of inductance.
The vertical
whip section was trimmed a little shorter bit-by-bit ( around 8-9mm,
or 1 MHz, at a time ) as the SWR was initially lower at 144 than
at 146 MHz ( so initially too long - as expected ).
The final SWR was 1.2:1 at 146MHz at a radiator length of 1195mm
- thus confirming the actual length correction factor was close
to 93% for my construction technique.
The final
whip assembly also produced a SWR of 1.4:1 at 50.100 MHz - undoubtedly
exhibiting a response as a slightly-base-loaded 1/4 wave whip on
6 metres.
I also tested
it at 439.0 MHz ( 70cm band ) and the SWR was about 2.5:1 and, while
not great, it would almost be useable if no other antenna was available.
Of course the issue on this band is that the major lobes will undoubtedly
be up at quite a high angle and, while suitable for accessing local
mountain-top repeaters, would not be particularly suitable for communications
at low angles e.g. to contact distant ground stations.
How
well did it work ??? Well
it is difficult to really do truly quantitative measurements of
one antenna against another because factors such as angle of radiation
are involved. My best response is : on my normal 1/4 wave whip on
a fixed antenna base on the car, one of the distant 2 metre repeaters
"showed" one bar on the signal level indicator - and an
occasional flicker up to a second - on my Yaesu FT-1802M. Unscrewing
the 1/4 wave and screwing on this 5/8ths, done within a couple of
minutes - and nothing else changed, either settings or physical
environment-wise - the FT-1802M's signal display rose up to around
"6 to 7" bars - and the FM signal quieting was certainly
better. I don't know what the FT-1802M display change means in terms
of actual signal levels in dBm, but using this antenna made an obvious
performance improvement. Subsequent time spent driving around through
the "same neighbourhoods as previously with the 1/4 wave"
revealed better signals from most of the repeaters.
If
you are looking to make / build your own two metre 5/8th's whip,
at least you found some relevant details - certainly more than I
did when I did the same search.
By
the way, have you ever heard of a "curly - whip " antenna
????
I used to use one of these on 2m mobile, and another one on 70cm,
and it is a similar concept ( half wave whip with base loading)
except the base coil is wound as a "one turn" inductor
with a slight overlap for matching.
I must see if I still have the dimensions somewhere in my filing
cabinet .....
Roy VK4ZQ started us using these locally back around the 70's...
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Email
from a web site visitor :
(29 June 2010)
Roll-over with
mouse for larger view
Experimented
with your plan for a 5/8 wave 2M antenna today, it worked very well.
I used a piece of galvanized wire from my garage, stiff material
used to bind things up. I started with a length 60 inches long,
made a very crude coil on one end, 3 1/2 turns about 1" diameter,
then inserted the end of the coil into a mag mount formerly used
as the base of a 1/4 wave 2M antenna. The coil & the whip are
one piece. I have an MFJ Antenna Analyzer 259B which basically made
my extremely crude efforts pay off. I kept nibbling pieces of wire
off the end of the whip until I got an SWR of 1.2-1.6 in the 144-148
MHz range. I did not attempt to stabilize the coil, the SWR varies
a bit as the whip waves back & forth, but it is always less
than 2.0. The height of the entire assembly from the base of the
mag mount to the tip of whip is about 52 inches.
I am able to reach some troublesome 2M repeaters in my area at my
transceiver's lowest power settings, settings that do not work with
my 1/4 wave antenna. My transceiver's signal strength meter shows
several more S-units on signals received from weaker repeaters.
Photo attached. The overhead wires are quite a bit above the top
of the antenna.
Not bad for about 1 cent worth of materials, if you don't count
scavenged parts and the expense of the antenna analyzer. 73 - Art,
KD8CGF
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My email
response :
Art
Good to hear from you. The main reason that I wrote that page up
on my website was that I was building a 2m 5/8ths anyway and I had
not been able to find much in the way of technical details on the
web as to how to do so. You will have noticed from my other web
pages that I now tend to document things so that it can provide
an insight for others, particularly with the introduction of the
simpler classes of amateur licences (eg our VK Foundation series
) and as less-technical people enter the hobby.
It doesn't really matter how the antenna is constructed provided
that the bottom coil characteristics approximates mine and that
the vertical radiator is around the same length. Galvanised wire
probably is not the best material because at VHF the conduction
is on the very outside of the wire (skin effect) and I doubt that
the galvanising layer has the lowest electrical resistance in the
commonly available metals.
Even so, it works for you similarly to what it did for me - and
mine was built entirely from bits I had here - i.e. nothing was
actually purchased to make mine either.
I might grab your text and photos to add to that web page just to
show that the construction details/trends I described do work for
others too.
73
Doug
PAGE NOTE
: You
don't have to have an RF impedance/antenna analyser to tune this
antenna. I did my adjustments with a SWR meter that worked to 150MHz,
using the lowest possible RF power to give a full-scale forward
reference value reading ( 5 watts ) and on a locally-inactive simplex
frequency for my testing.
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