The choice of HF antenna is vast so
which shall I go for?
It's easy if you are wealthy with acres
of real estate, multi-element horizontal beams mounted a quarter
wavelength+ in the air over rotators. For the rest of us, the 99%,
it's not so easy. I live in a typical semi with an area of around 35m
x 10m with close neighbours so antenna options are a challenge.
Over the last eight months, since
returning to the hobby after several decades absence, I must have
explored many of the available HF antenna options Here's a simple
summary of my initial conclusions.
- Magnetic Loops
For some strange reason, maybe because
I saw this as a difficult challenge, I jumped in at the deep end and
began my return to amateur radio by building a couple of magnetic
loops.
A magnetic loop simply comprises a
conducting loop 0.10-0.25 wavelengths in circumference with a 25-50mm
gap across which a tuning capacitor is connected. The loop can be fed
by several methods, the simplest of which is a coupling loop 20% of
the main loop diameter, mounted close to, but not in electrical
contact with the primary loop, and opposite the tuning capacitor.
Fig.1 Magnetic Loop
The key requirement for an efficient
magnetic loop antenna is that its resistance must be extremely low to
display the high Q necessary to efficiently radiate RF energy. This
consideration leads to the high cost and complexity often associated
with this antenna. To keep resistance losses low the loop ideally
needs to be constructed from relatively large diameter (5-20mm)
copper tube, the tuning capacitor is usually motor driven and needs
to be of the vacuum type, or at the very least the split stator
variety, and all electrical connections need to be of very high
quality.
As it turned out the route I chose, to
evaluate the concept of a magnetic loop as simply and as cheaply as
possible, worked rather well whilst still guaranteeing the very low
resistance required. OK, this initial project didn't incorporate a
motor driven capacitor for tuning, but I'm not sure one was needed as
the following explains.
The first hurdle, to construct a 1m
diameter circular loop using 15mm copper plumbing pipe, was much
easier than I expected. A pipe bending spring was purchased from eBay
for £7.50, and a 3m copper tube (cost £15) was bent into a near
perfect circle using a wooden former mounted in a workmate.
Instead of connecting a variable
capacitor across the loop gap a simple fixed capacitor was produced
by using a gas blow torch to solder a 70cm length of RG214 coax
across the loop gap. The target capacitance needed for this loop to
resonate on 14.200MHz , as specified by an online loop design
calculator, was 64pF. RG214 coax has a capacitance of around 100pF/m,
so it was a fairly straightforward process to tune the loop by
trimming the coax to the optimum length.
Fig.2 Simple fixed frequency
magnetic Loop
Having a fixed rather than a variable
capacitor wasn't a big problem. The finished loop tuned from around
14.170MHz to 14.230MHz. If one decided to use this design for a
permanent installation it could be tuned to cover most of the cw or
data portions of 20m, and two such loops mounted side by side, with a
suitable gap between them to prevent interaction, could cover a
sizable portion of the ssb segment of this band whilst still taking
up less space than a dipole. Also, the top of a 1m loop only needs to
be mounted 2m above ground level, a significant saving on the 5m
minimum height needed for a dipole.
So how did this loop perform. I now
regret that my trials with this loop were somewhat limited because I
was very keen to erect a multi-band antenna to get fully operational
while designing and then building a multi-band loop incorporating a
variable capacitor as a longer term project. On receive the loop was
very promising, not least because it's sharp tuning characteristic
rejected a lot of the QRN ever present in the urban environment. The
wire antennas that I went on to build often had noise floors with
s-meter readings of 5-7 on some bands. This loop had a noise floor of
1-2 which I'm sure would have given it great potential with weak dx
signals. Comparing the loop to a temporary 20m ¼ wave vertical with
8 buried radials, most signal strengths were the same or similar.
On transmit, only a couple of QSO's
were completed (the antenna wasn't up long) with 59's recorded both
ways into Europe.
Overall, I rated this experimental loop
quite highly. It was cheap (<£30), much easier to build with the
necessary low resistance than I expected, and very low visibility for
the neighbours, but on the downside my limited evaluation didn't
promise much better performance than that of a ¼ wave vertical. Then
again, mounting it higher up and on a rotator might have been a
different story.
I've now purchased a lot of the bits
needed for a permanent tuned version, and my multi-band antenna
installation is complete, so all that's stopping me proceeding with a
mk2 version is the biggest hurdle of all – motivation.
- ¼ wave verticals
I erected ¼ wave verticals for 10m, 20m, and 40m. All were installed
with eight buried ¼ wave radials, and much to my surprise worked
well even though they were positioned close together (2-4m spacing).
Many QSO's were recorded, including quite a few with North and South
America and the Caribbean on 40m, and I would have been happy to use
them on a permanent basis. However, no VK/ZL contacts were made so
there was certainly scope for improvement. I also thought the 40m
vertical was too high for good neighbour relations, and wanted to be
active on 80m/160m so my attention focused on to horizontal antennas.
The 10m and 20m verticals have been retained as “back up”
antennas and for comparison against newer antennas.
- Multi-band end fed inverted “L”
The successful version of this antenna that I finished up with was
32.6m long, 15m high at its highest point, and fed remotely at the
bottom of the garden using an old LDG automatic atu mounted in a
water proof box.
Attempts to feed this antenna with various baluns and ununs using a
coax feed and an atu in the shack had proved universally disastrous.
The SWR's were reasonable on a couple of bands but high on most.
Losses in the 15m coax feed must have been high, and on receive the
antenna sounded “dead”, often without the characteristic rise in
background noise one expects to hear when an antenna is tuned to
resonance. I learned a golden rule at this time.
NEVER FEED A MULTI-BAND ANTENNA WITH COAX UNLESS THE ATU IS AT THE
FEEDPOINT.
This may not be a universal rule, for example a short length of very
low loss coax may be tolerable, but as a rule of thumb I think it's a
rule I will stick to in the future.
With the atu in the correct position, at the feed point, I was much
happier with the performance of this antenna. It tuned up on all nine
HF bands although several were 1.9 SWR. Comparing it with the
10m/20m/40m ¼ wave verticals that were all still up when it was
erected, the inverted “L” was perhaps a little down compared to
the 40m ¼ wave, but was generally similar in performance or
sometimes better, compared to the 10m/20m verticals.
A minus with this antenna was the need for an extensive buried
radial system.
- Ladder fed multi-band doublet
My time with a remotely fed / remotely tuned inverted “L” was cut
short when the outside atu stopped working. Faced with the prospect
of spending several hundred pounds on another outdoor atu I decided
on the cheaper option of a ladder line fed doublet, which would have
the advantage of not needing an extensive radial system to maintain,
and could be tuned by a manual atu in the shack without the worry of
coax feeder losses.
The commonly employed 65ft x 65ft doublet was erected, fed with 38ft
6in ladder line. To fit this within my boundaries it was necessary to
allow the outer 15ft of each top to hang down vertically from the end
supports. Each doublet half and ladder line half was made from one
103ft 6in length of insulated wire so waterproofing should never be
an issue with this antenna as there are no joints anywhere. The
ladder feed, spaced with pvc tubes giving a 40mm spacing and 450 ohm
impedance, is fed directly into an MFJ941E manual tuner in the shack.
This antenna has only been up a few days, and I've already received a
58 report from North America on 40m, so performance seems just as
good as that of the previous inverted “L” antenna. On receive,
it is 2-3 S points up on the 10m ¼ wave vertical, very encouraging.
Ten HF bands tune up easily too, so hopefully I can now spend more
time in the shack and less time on the roof in the future.
So which antenna is the best?
To be honest, I'm not sure. It goes without saying, none of these
antennas can match a beam yet they all give similar performance with
which I'm more than happy. After all, it must get boring working
VK/ZL every day. For mono-band use I quite liked the 40m ¼ wave
vertical, but for all band coverage it has to be one of the
horizontal wire options, and from a cost and maintenance point of
view, I guess the doublet becomes the clear winner.
What's next to try?
Perhaps a quad or a loop might be worth looking at next, but I think
I'll stop there for the time being and concentrate on operating.
