If you listen to the DX activity on the Low
bands, you'd be forgiven for thinking that you're wasting your time unless you
have a 4 Square Array, 20 acres of Beverage antennae, a huge Amplifier, and a
neck like an anvil..
We probably aspire to having the lot, but
thankfully, we can do a passable job without any of them. Those of us,
who live in average homes, with average sized gardens, and small to average
bank accounts, can be surprisingly successfull on the Low Bands, once we
understand what the minimum requirements for success are..
Really, The main area of
importance is the antenna farm...!!
On the higher bands, the
same antenna is usually used for
both TX and RX . The Low Bands are different, in that one
good TX antenna is probably adequate. If the TX antenna is
relatively inefficient, we can compensate by using more power. But for RX, we require antennae
that have the very best signal-to-noise ratio (SNR) that we can muster.
It is also very advantageous if these antennae are directional. Sensitivity in
the RX antenna is not really that important, so long as the SNR is
excellent.....A good low noise amplifier (preamp/LNA) can give us all the
sensitivity that we need.
First of all I must qualify my
remarks by stating that ALL antennae benefit from a good ground under them. ( except the Beverage... "The exception
proves the rule" )
One area of my garden
has lots of radials, buried wire mesh etc.. I have run
many verticals on this site of various types... 1/4 w/l verticals, inverted
Ls...trapped and otherwise, and they all worked well. The Verticals have never
been more than 50ft high, so various forms of loading have been used to bring
them to resonance. Typical of these would be my 80&40mVertical
. Being a fan of L B Cebik
W4RNL and his excellent site at Cebik.com
, I learned that the MOST efficient form of loading was
"capacity hat" loading. I built various antennae using this method,
and they were the most successful 1/4 w/l verticals that I've ever built.
I found an
interesting project on LBs site consisting of a triangle of three vertical
Dipoles . Shortly afterwards I discovered Don K2KQ's excellent antenna which he
calls his Double
L . This is probably one
quickest lowband antennae to deploy, and is very effective. It does not have
the overhead null of the typical vertical, but is a good DX antenna, and can
also be used for relatively local QSOs.
I then ran across
Way to Look at Vertical Antennas" by Rudy Severns N6LF (March 1999
QEX)... This was the "icing on the cake" ... Not alone had Rudy got
all the practical stuff that I needed in the article, he also had the
gain figures for his vertical dipoles compared to traditional ( 1/4 w/l) verticals
(A drawing to show the
progression from standard dipole, to Capacity hatted, end loaded, vertical
As the Capacity hats get
larger, the resonant length shortens
In the article, Rudy models a
traditional full size 80m 1/4 w/l with 4 elevated radials. He compares this
with a number of "H" antennae, which vary from 30 ft to 120ft in
height. For the full size 1/4 w/l vertical, he predicts a feed impedance of 35.7 Ohms, a
2:1 SWR bandwidth of 175kHz, a peak (take off) angle of 22 degrees, and a gain
of +.21 dBi. Using his tables, my "H" dipole comes up as
having a centre feed impedance of 45 Ohms, or 65 Ohms if fed at the
bottom. A 2:1 SWR bandwidth of 170kHz, a peak angle of 23 degrees, and
gain of -- .25 dBi
You can see from this, that my
50ft "H" has almost exactly the same bandwidth and peak angle as
the full size 1/4 w/l vertical, and the gain is only down by .5 dBi..
I have come to the
conclusion from my experiments, readings and observations, that a capacity
hatted vertical dipole, a few feet over ground, is LESS COMPROMISED than a 1/4
w/l vertical of the same height fed against a less than perfect ground. Let's
face it, MOST amateur's ground systems are mediocre at best. Also, the dipole is
easier and cheaper to rig, and is two dimensional..Very important in my situation, as I cannot run out radials on my
neighbours property. Or, to quote W4RNL..
"Since only a handfull
of hams can ever have 160-meter antennas high enough to yield a low angle DX
signal, more practical are vertical arrays such as yours. Vertical
dipoles with hats (or Tees) save a plethora of wire needed by monopoles."
W8JI in a mail to
the Topband reflector on the subject of small antenna efficiency, sums it up
"The most efficient way
to build a small antenna is to use as much capacitance as possible at the antenna ends, and to keep the high voltage
areas away from earth or lossy media.
It is always a tough project to build an efficient small antenna,
but the bottom line is you want the highest current over the largest straight area
possible and the open ends away from things that might be lossy. Using high Q
coils can actually be much more efficient than folding wires, but gives less
bandwidth. Capacitance hats at the open ends always help bandwidth and
dimensional, I can rig the dipoles along my fence line, and this leaves
the original site, complete with it's ground system , available for the use of
my K9AY loops..
(Drawing of the original pair of
phased 80m vertical dipoles at EI7BA)
I'm now using 45ft dipoles. loaded top and bottom (cap hat wires) on 80 and
160m, suspended from a 60ft high timber mast, as in the drawing above.
I have done side by side comparisons with my Inverted L, and various 1/4 w/l verticals
that I've built. They compare very favourably indeed, with the vertical
dipole usually a tad better than any of my 1/4 w/l verticals both on TX and
RX. For some reason, the dipoles are quieter on RX than the others...'dunno why...Not that it's an issue here, as I use a K9AY on RX when the going gets tough..
The dipoles are much quicker to erect, as they're only 2 dimensional (no ground system). Mine are erected along the fence between two fields on
my neighbour's farm. Being no wider than the fence itself, the antennae do not
interfere with the agricultural activities in any way. This means that my
farmer neighbour is quite happy to let me put up as much wire as I like...
( view to the
live on the coast, we get strong winds in the Winter. To
"streamline" the antenna, I now feed the dipoles at the base of the
IMPORTANT... Because the
dipole is now fed asymetrically,
we must use a good choke balun at the feedpoint. At these frequencies,
we must use a
high permeability ferrite core. I use a FT-240-61 core, wound with as many turns as possible of RG58 coax ( I think I got about 12 or13 on mine).
Wind on about half the turns, then cross to the opposite side of the core, and wind in the opposite direction for the remaining turns.
If you've got any large
ferrite cores in your junk box, and you don't have a clue what permeability
they are, here's a quick check that I use.
Wind two tight turns
around the core with insulated wire.. Connect the ends to the antenna socket
of your Antenna Analyser (MFJ 259 etc) Switch function to read
inductance, and set the frequency to somewhere
If the meter reads any
value of inductance, then that core is probably useful down as far as 160m.
I also use the same method to check small cores for use as 9:1 traffos for a
Beverage, K9AY etc.
Here's a quick way to check a choke balun.
If flopping the
cable around on the ground, or handling the bottom end of the choke has no effect on the SWR, then it's
probably working effectively. If the SWR varies with this treatment, then it's
NOT working properly.
uses a drum of coax as a choke on his 160m array....Overkill..? perhaps.. but
On the commercial front, Force 12 have
stirred up quite a bit of interest with their Sigma
Vertical antennae. This antenna looks uncannily like the vertical
dipoles that W4RNL has had on his website for years. Take a look at what they
have to say about vertical dipoles..
Their 80m Sigma has a 36 ft
vertical section, has a 70kHz SWR bandwidth, and I'm
sure it is an effective antenna. If you've got a handy 50ft or higher tree on
your property, the wire version should cost $50.00 or less, and with the
higher vertical section should be even more effective..
The next modification that I intend to make to my Vertical Dipole, will be to
follow LB's suggestion, and use extra vertical wires spaced about 4-6 inches.
This should increase the bandwidth, and general efficiency (less resistive
losses) of the antenna.
My RX Antenna..
the low bands, the average amateur receiver has excess gain, and so a low-gain
receiving antenna with a rear null is beneficial.. Not only does it
improve the signal to noise ratio and remove rearward QRM, but it also reduces
the occurrence of front-end-overload from QRM, QRN, and from nearby AM BC
stations in the MF range. Hence, gain must be tailored to the situation--which is not the same on 160 as it is on 20
shape--both horizontally and vertically (i.e., the whole 3-D pattern)--becomes
more important than gain itself."
The above quote from W4RNL is
an excellent synopsis of the requirements for good Low Band RX performance.
For an "in-depth" discussion on the topic go to Tom
W8JI's website and read his Receiving
do of course, use my TX antennae on receive also.. However, The single most
important addition to my DXing armoury is definitely
my K9AY loop array...
Gary Breed K9AY, did us all a huge favour, when he published that famous
article in September 1997 QST Magazine. I could "wax
lyrical" about the effectiveness of this antenna, but I shall
resist the temptation, as it has all been well covered before.. I will just
mention that the K9AY is one of a number of small and effective
receiving antennae, such as the EWE, Flag, Pennant, K6STI etc. that have
become very popular.
There is a wealth of information to be found on the web about all of these
antennae , on some really excellent websites.. Your first visit
should be straight to "the horse's mouth " at
Gary Breed's K9AY site . Other
sites that you should visit include Mark
Connelly WA1ION , Tom
W8JI , Flag and Pennant Antenna Compendium
Core DX , and Wellbrook
products to mention but a few..
I have made two physical
modifications to the original K9AY design..
( Drawing of my K9AY variation..Higher bottom wires = fewer
I have raised the bottom of the loops, to a more comfortable height for
lawnmowing etc.. and I have added a further two relays to the Control box.
These relays switch in extra resistors in parallel with the main terminating
resistor. This allows me to have optimum values for both 80 and 160m, without
the complexity of controlling a Vactrol.
Just two examples of my experiences with my K9AY..
The 10th August
2000.. FR/F6KDF/T on 1.824mHz. Tromelein Island DXpedition. QRN S9 on the TX
vertical. EU QRM almost crunching the front end on my FT1000MP.. Absolutely
no copy on the DX at my end. On switching to the K9AY Loop, the QRN dropped to perhaps
S5. With the null pointing at EU, FR/F6KDF/T is now readable.
I worked him,
in spite of the competition from the EU "big guns" 5 5 9 both ways.
I'm not a contester, but I do like to give out as many "mults" as I
can on the Low Bands. With the EUs in full cry, it's very hard to copy
Stateside on the TX antenna when the QRM is way over S9. On switching to the
K9AY the noise level drops dramatically, the signals also drop a few S points,
but with the extra 20/30dB null now pointing at Europe, the Stateside
stations, although weaker, are now "armchair copy".
I do not
mention Beverages, 4 square arrays etc on this page, as it's aimed at the
Amateur who has no possibility of erecting such large antennae.. I hope that
it may give some encouragement to those who (like myself) have limited space