Vertical
antennas :
Ok, that leaves a "trap-vertical" style antenna if I want
to cover all main HF bands. I have a trap vertical - a CE-5SS {
don't recall the manufacturer and can't find any info anywhere on
the WWW } - that I bought new back in the 1980's.. It had a "trap
failure", due to corrosion, back about 10-15 years ago and
I had to cut the aluminium trap shroud tube in half to get it apart
- to repair the trap. I had never really been happy with it since
then but it had stayed attached to a RG213 coax feed into the shack
- and rarely used. It was time to re-evalute the CE-5SS because
it at least covered the HF bands of interest - 3.5 / 7 / 14 / 21
and 28 MHz. Have you ever tried to make a trap vertical "work"
by mounting it on a short piece ( about 1.8m ) of masting tube set
vertically on top of (but not in) the ground. It doesn't work. It
really requires a ground plane, whether that be the "actual
earth" or a "synthesised earth". Now I may not have
a piece of pipe in the ground where I am going to "field day",
each and every time. I would have no idea how far in the ground
any pipe I 'luckily found' would be, the actual earth resistance
etc.. so it would appear that a system of radials would be required
- and thus make the installation "independent of earth".
Actually
the documentation on the CE-5SS is a bit "confusing" (
make that wrong !) : it says that the bottom trap is a dual
unit for 15 metres and 20 metres and that the inherent resonance
of the lower-most tubing section is around 28.6 MHz. You adjust
this 'length' for best SWR on the 21 MHz band. In a way that is
true - the 21 MHz 'active' section is the base tube plus the 28
MHz complex trap impedance 'below -resonance' plus the "outer
tube length" of this lower trap and the trap is tuned to 28
MHz - not 21 MHz ! The upper/other trap in this single 'trap assembly'
is tuned to 21 MHz - not 14 MHz. The adjustable tube length above
the trap is adjusted for best SWR at 14 MHz / 20 metres - so it
cannot be a 14 MHz trap ! At the top of the assembly is a long PVC
tube which is capped by two adjustable protruding 'whip elements',
one for 7 MHz / 40 metres and one for 3.5 MHz / 80 metres. Back
some years ago, I did look inside this PVC tube but don't recall
exactly what I saw - I subsequently re-sealed it fairly well and
am reluctant to open it up again. Obviously it contains 'traps'
at 14 and 7 MHz but also must contain inductive loading coils to
allow such a short whip tip ( about 95 cm ) on each band.
The CE-5SS
product sheet has an interesting footnote : It starts off with "Radial
Length Chart", tabulates radial lengths for the various bands
( a 1/4 wave at each +/- adjustment alowances) and then states "
Only one radial wire per band should be sufficient ". I thought
I would have to test this concept, right or wrong. I cut an insulated
stranded wire for each band as per the tabulated data (i.e. their
'cutting chart'), attached it to the mounting plate at the base
of the whip, 'drooped' it down along the ground, and checked the
results. I started to see a reduction in SWR on 3.5, 7 and 14 MHz
( under 2.5 : 1) but 21 and 28 MHz were still bad news ( > 5:1).
Drawing these radials up towards horizontal (rather than drooping)
lowered the SWR ( < 3:1) so I knew it was possible to use just
one radial wire per band - "IF" I could get them up around
horizontal. Problem is that with field days, you really don't know
if you will have a tree to tie the ground plane radials off to.
Thinking cap time again !
I delved
into my 12.5mm aluminium tubing stocks and created a 28 MHz radial
arm as per the 'cutting chart' and attached it just below the trap
vertical base with a u-bolt. I then went and re-checked the SWR
on 28 MHz - it was down to about 1.25:1 so I thought, let's try
the same for 21 MHz. More aluminium tube, attached to the same pair
of exposed threads on the same u-bolt but protruding the 'opposite
way/direction'. The SWR on 21 MHz was now becoming acceptable (
< 2:1) so I knew that I definitely could get away with just a
single radial per band ! More tubing later and I had a radial for
14 MHz attached. You know, it really didn't make much of an improvement
over the drooping wire - plus it was 5.3 metres long - so very 'saggy'.
If I was going to go that way, I would have to allow some supporting
frame just to hold the radial more-or-less horizontal.
I have a
full set of HF helical whips (Mobile One M-80 to M-10, 3.5 to 28
MHz, non-WARC bands, 5/16" 'Australian thread' base) for the
front of the 4WD - would it be possible to use them as "short
'tuned' radials" in lieu of the long-ish aluminium tube radials
( for 21 & 28 ) and the wire radials for 3.5, 7 and 14 MHz ?
A search of the web (WWW) using Google didn't reveal anybody had
written-up doing something like this ( using them as tuned radial
arms / 'ground planes' ) before - so I was moving into uncharted
ground. I had some old 5/16" antenna bases so drilled 4 clearance
holes between the u-clamps in the trap dipole vertical mounting
plate, used only the threaded sections of the antenna base and used
the 'top nut' plus some flat washers to secure the shafts through
the holes. I now had 4 by 5/16" threaded sections available
to screw on a selection from the range of mobile HF whips, offset
diagonally on each side of the vertical plate. Initially I removed
all other radials and started with just the 10 metre / 28MHz whip,
screwing it on one of the threaded pieces, checked the SWR and found
it reasonable ( < 3.0:1) but not brilliant. One thing I did notice
was that the actual tuning of the 10M whip affected the overall
resonance on that band. I had to adjust the whip length/tuning (fortunately
this series has adjustable 'tuning' tips) to get the SWR to drop
in the right segment of the 28 MHz band.That set the pattern too
for the other bands. I then added the 15M whip onto a threaded segment
on the opposite side of the vertical mounting plate, checked SWR
there, found it around 2.5:1 to 3.0:1 - so a similar scenario to
the 28MHz results. The results with the 14 MHz whip were better
and resulted in a lower SWR ( < 2:1) and again, tuning of the
whip length/resonance affected where in the band the SWR dip was.
The 7 MHz whip produced similar results to those obtained on 14
MHz. The very narrow bandwidth of the 3.5 MHz whip meant that the
SWR dip on 80 metres was very sharp and very pronounced and was
deemed to be too limiting for actual use.
On going
back and checking the SWR across the bands 28 MHz down to 3.5 MHz,
I was happy with the results on 7 and 14 MHz but the other bands
simply did not produce acceptable values. After I removed both the
28 and 21 MHz helical whips and re-attached the aluminium radial
pieces for those bands, the SWR on those was considered acceptable.
On re-connecting the 'wire radials' for 3.5, 7 and 14 MHz, and leaving
the 7 and 14 MHz helicals in place, the SWR across the appropriate
contest 'phone' sub-segment of all bands was 1.5:1, or better. The
full SWR results have yet to be recorded and charted.
I don't know
whether your particular transceiver uses a single antenna port for
HF + 6 metres (50 MHz) but many do. Certainly mine does so I thought
that a little more experimentation might be in order to see if I
could 'add' 50 MHz coverage. Using a right-angle 'trucker's mirror
mount' as a mounting base, I attached a thin 1.3 metre long stainless
steel whip immediately above the vertical mounting plate's top insulator
and proceeded to slowly move it up looking for a low SWR on 50 MHz,
preferably at 52.5 MHz too. I found a position where the SWR on
50.1 was about 2.5:1, about 1.2:1 at 52.1 and 1.5:1 at 52.5 MHz.
Final length from tip to base was close to 1.5 metres, so a quarter
wave at 50 MHz. One thing I noticed was that the SWR was changing
as the whip fluttered about in the wind so it was going to have
to be 'restrained'. A suitable arrangement was made from two sandwich-style
antenna element insulators which were 'tension mounted' on the pipe
some 150mm below the top of the 50 MHz whip. This resulted in negligible
movement in the wind and a complete lack of SWR change with wind.
Attaching a 50 MHz radial arm did not affect the SWR at all so was
not considered necessary in the final configuration. Obviously one
of the other radials was doing the job for 50 MHz too ! That gave
me the added bonus of (particularly) 6 metre FM coverage (52.525),
and while not 'flash' at the 50.1 SSB DX segment, it should work
there too - as necessary. The thing that was achieved by this was
the ability to work "6 SSB or FM" without needing to use
a coaxial switch to go from HF to 6 metres & vice versa...
The result
: a HF antenna covering the non-WARC bands from 3.5 to 50 MHz, reasonably
portable, effectively "ground independent", quite light
and readily erected by one person.
The final
step was to go ahead and mark each and every flexible/sleeved joint
with a red permanent marking pen so that I knew what lengths were
in use at the time.
Follow-up
experiments :
The idea
of the separate wire, extra tubing radials and using the HF helical
whips for the ground plane was irking me a bit.. It was going to
take too long to get the "radials" ready when setting
up for FD operations. I was sitting in a comfy chair out in the
yard a couple of days ago - trying to catch a bit of breeze to cool
down in the summer heat and was looking up at my Diamond W-8010
(W8010 for Google) multiband trap dipole { 3.5, 7, 14, 21 &
28 - now modified to also cover 24, 18 &
10 MHz } and thought " I could always try a single 80/40/20/15/10
trap-style wire element as a radial system". So it was an idea
born..... that came to reality today ( 8 March 09, a week to go
to the JMFD).
Many of you
who build multiband wire antennas will have "tripped across"
the Coaxial
Trap Design software by Tony Field VE6YP - { download:
coaxtrap.zip & read the HELP ! } and I had used it sucessfully
before when doing the development of the two
traps at 18 and 24 MHz for the W-8010 modifications - so I started
measuring the outside diameters of the PVC conduits I had available
here. I had 4 sizes : 16, 20, 38 and 40mm OD - so I started with
the 16mm dimension and that was workable for 28 and 21 MHz, the
38mm seemed reasonable for the 14 and 7 MHz traps. All PVC tubes
used were cut to 75mm lengths, regardless of diameter. I have a
reasonable stock of Belden RG174U miniature 50 ohm coax so jotted
down the dimensions of coil turns, coil length, and coax length
for each band using that cable. Just remember, this is a resonant
radial system - not a vertical radiator that needs to use
large coax to handle high power. Even so, it will certainly handle
100W+ PEP, maybe up to 400W PEP - I just have no need to try it
at that high a power level !
I started
with construction of the 28MHz trap and made it up with the recommended
length of coax - and it resonated about 32 MHz instead of around
28, as measured with my GDO. Then I recalled that you have to start
with about 10% MORE coax than that calculated - that was something
I found when making the 18 & 24 traps and had forgotten until
now. I also put a slot at one end of the former for the RG174 to
go through so that I could expand the actual coil turns to do fine
freqency adjustment. I started at the slot, wound the turns and
then drilled an 'exit' hole for the other end of the RG174 to pass
through. Did the soldering inside the PVC former - carefully so
as not to melt any PVC - and tested the re-wound trap. It finished
around 28.5MHz (after adjustment by spreading turns) so time to
move on to the next one. The 21 MHz trap came up reasonably easily,
then the 14 MHz one on the 38mm PVC former, and finally the 7 MHz
one - also on a 38mm former. The trap wire ends were secured onto
solder lugs that were fixed via holes in the end of the PVC formers
with 4mm screws (25 mm long), flat washers and nuts. The final constructed
values against the calculated are as follows :
| Freq |
Design |
Final
Turns |
| 28.1
MHz |
5.98T,
20mm dia, 1.52cm long, 44.92cm coax |
6.5T |
| 21.1
MHz |
7.66T,
20mm dia, 1.95cm long, 56.79cm coax |
8.25T |
| 14.1
MHz |
5.8T,
38mm dia, 1.47cm long, 76.41cm of coax |
7.75T
( yes, 7 3/4 turns ) |
| 7.1
MHz |
10.14T,
38mm dia, 2.57cm long, 131.63cm coax |
10.125T |
Please
note : these coax traps do not seem to need to be ultra-fine tuned.
They are low-ish in Q and provided they are close to the desired
frequency, the actual resonant frequency is not super-critical.
Fine tuning is done by spreading or compressing the turns on the
PVC former - hence my slot at one end for the RG174 to pass through...
Once tuned, use some PVC tape (or something better) to hold the
turns in position.
In practice,
the 4 traps took me about 1.5 to 2 hours to create and required
just a screwdriver, cutters, drill + 2 bit sizes, soldering iron,
solder and an accurate GDO (grid dip oscillator, though mine is
transistorised) apart from the hardware : PVC formers, 4mm screws,
nuts, washers, solder lugs and RG174U cable.
| The
following images are of the 7MHz trap (after being taped up),
the others are much the same idea (even if a different size
former) .... (mouse over the images for larger view) |
|
|
|
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| This
image shows the 38mm PVC former after being taped up. A close
look on the larger image will reveal that the turns are spread
apart towards the closest end. Note the use of 4mm screws, nuts,
etc and crimp lugs (soldered too) for the incoming, outgoing
and tail wires. |
The
inside view reveals the slot used to make the tuning easier
-far easier to expand the "coil" as against a small
fixed hole.. And by making the coax around 6 - 10% longer, you
will almost always have to "spread the turns" ! |
The
overall image shows the "40 metre" tail attached.
In practice, it simply 'droops' down from the trap. |
I might eventually
fill the PVC formers with something like the expanding poly-foam
fillers (available in spray-style can, used for moulding around
and shipping goods) to provide some additional robustness and weather
protection, rather than trying to find end caps...
The traps
were ok so now time to do the wire segments in between. The basic
1st segment ( A ) is a 1/4 wave at 28 MHz so that was easy - set
it at 2.56m - as "close enough' - and work from there. One
of the things I did like about the Diamond W-series design was that
the final tuning was done with droopy "tails" that were
trimmed with cutters to set the final resonance point - rather than
adjusting the actual spacing lengths between the relevant traps.
Oh, so much easier...
The
wire segments were terminated onto crimp lugs (subsequently soldered
as I detest crimp-alone connections) which simply slipped over the
4mm trap termination screws and were retained by an extra 4mm nut.
Where tails are required (except the 80m one), the tail wires are
also terminated on a solder lug and again held in place by yet another
4mm nut. If you trim the tail too far, simply remove it by unscrewing
the nut and replace it with a new tail with a longer wire. No length
adjustments between the traps are necessary during tuning (at least
on mine, though they might be on yours - if so, shorten A &
C - and add tails !). { Recommended : initial tail lengths be about
300mm, trimmed back with sidecutters during the tuning process }
Remember to tune "down in frequency" ie. adjust 28 MHz
first, next 21, next 14, next 7 and finally 3.5 MHz.
In this design using tuning tails, it is not as critical to
follow the "tune down" process as it would be on a design
"without tails", as the lengths between the traps should
not change in this style - only the tail lengths !
Final
spacing values turned out as tabled, all using insulated multi-strand
copper wire (if uninsulated used, add about 2% to each length) :
| Wire
position |
Drawing
Dimension |
Length |
Adjustment
tail, connected to "inner end" of the outer trap |
| Trap
vertical 'earth lug' to 28 MHz trap |
A |
2.560
m |
28
MHz tail - None required ( probably this section could have
been shortened by 20cm or so and a 'tail' added to make setting
up easier ) |
| 28
MHz trap to 21 MHz trap |
B |
25
cm |
21
MHz tail - 175 mm tail (adjusted for desired centre freq on
21 MHz eg 21.200 ) { 21 MHz wire length is effectively A + 28Trap
+ B + 21 tail length } |
| 21
MHz trap to 14 MHz trap |
C |
58
cm |
14
MHz tail -None required ( probably this section could have been
shortened by 10cm or so and a 'tail' added to make setting up
easier ) { 14 MHz wire length is effectively A + 28Trap + B
+ 21Trap + C + 14 tail length ( nil in this case) } |
| 14
MHz trap to 7 MHz trap |
D |
3.20
m |
7
MHz tail -220 mm tail (adjusted for desired centre freq on 7
MHz eg 7.090 ) { 7 MHz wire length is effectively A + 28Trap
+ B + 21Trap + C + 14Trap+D + 14 tail length } |
| 7
MHz trap to end |
E |
4.475
m |
3.5
MHz tail -190 mm tail, drooping from egg insulator termination
(adjusted for desired centre freq on 3.5 MHz eg 3.590 ) { trick
on the 80 tail, don't cut it off - just twist/wind it back on
itself back up to the egg insulator to provide a length cancellation
effect } |
Note : Below
their individual resonant frequencies, the 'traps' appear inductive
thus shortening (by inductively loading) any subsequent 1/4 wave
wire length.
Once it was
"tuned" to the centre of the desired segments, it was
time to go through and do SWR checks again and see how good/bad
it was - in an overall sense.
| Band |
Freq
of actual SWR min (KHz), SWR |
2:1
SWR point - Low, High (KHz) |
3:1
SWR point - Low, High (KHz) |
| 3.5 |
3603,
1.12:1 |
3588,
3618 { 30 KHz wide } |
3575,
3628 |
| 7.0 |
7064,
1.2:1 |
7030,
7090 { 60 KHz wide } |
7005,
7108 |
| 14.0 |
14185,
1.1:1 |
14070,
14310 { 240 KHz wide } |
<14000,
>14350 { 14.0 = 2.8, 14.35 = 2.2 } |
| 21.0 |
21160,
1.3:1 |
<21000,
21380 { 21.0 = 1.8 } |
<21000,
>21450 { 21.0 = 1.8, 21.45 = 2.5 } |
| 28.0 |
28420,
1.0:1 |
<28000,
>29000 { 28.0 = 1.75, 29.0 = 1.75 } |
<28000,
>29000 |
The whole
creation process took about 1/2 a day - that included building the
4 traps, determining initial wire lengths (and subsequently trimming
these), tuning the tails and doing an SWR run across each band.
As expected,
the narrow bandwidths on 3.5 and 7 MHz will be caused by (1) the
traps on the vertical itself (2) that bandwidth modified by the
radial trap bandwidths. Even so, provided I centred my 80/40 field
day operations around 3600 and 7070, this antenna arrangement should
be quite a reasonable option.
Of course,
these assembly details could be "mirrored" on a second
"trap wire" to become a 'low power' 80, 40, 20, 15 &
10 metre dipole with a 1:1 centre balun and single coax feeder....
Please note
: the "trap radial" is connected at the base of the trap
vertical itself, some 1.8 metres above ground and is run out horizontally
- effectively STAYING around 1.8 metres above the ground. I realise
that this will give some horizontal component to the radiated signal
- and also attribute some lobe directionality but if I accept that
the radial should NOT be run in the direction that I wish to primarily
communicate in { it will be a minima due to end effect } - it should
work fine. For example, running the radial East or West would give
good results to North and South. I don't have any real need to run
a "lobe" computer analysis on it as every time it gets
installed, it will be different...
I can now
put the "trap radial" in the boxes with the 10m nylon
guy ropes of the trap vertical kit and leave my HF helical whips
available to use on the 4WD whenever I like... a much better solution.
Quicker too, as there is only one "wire" to run out and
tie off....
{ Guy ropes
??? - yes, you never know where (or for how long) the trap vertical
will be erected, and taking into account severe storms, cyclones,
...., it pays to prepare early !
In reality, I should never require the guys to be installed because
the installation method is reasonably stable - but you never know
what will happen in a field day setup ! }
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