Rockloop Antenna @dl1gsj
A magnetic loop made just of wire?
Years ago I started experimenting on magnetic loops & Co;
I like the idea of narrow resonant aerials. When I saw the
rockloop I never believed it might work (Ohmic resistance of the
wire). Nowadays, travelling around a lot, I was looking for some
antennas fitting my luggage. Again I found W9SCH's
rockloop.
Thoughts to the original design
If I interpreted the original design correctly, the rockloop is
constructed of a radiator wire, a transformer and a tuning capacitor.
The
transformer, as implied in
G3YCC's description , seems to be made of a different (at least
another) wire than the radiator. Following the calculations that
can be done on magnetic loop antennas, one knows about the extremely
low radiation resistance of these loops. This requires the total Ohmic
resistances to be as low as possible. To gain the lowest possible Ohmic
resistance, I decided to use a larger dimensioned toroid core to
fit five windings of the radiator wire on it (no solder joints, lower
losses).
Materials
- about 4m insulated copper wire
- 2 bamboo pole 1.2m long
- toroid core of your choice
- variable capacitor
- bunch of cable ties
- insulated wire for the primary transformer winding
- BNC connector or coax cable (or both...)
Dimensions
The four sides should have a length of 80cm, this
will enable
you to resonate the antenna up to the 15m band. If there are needs for
the
higher bands one should go for shorter sides. The transformation ratio
was suggested by W9SCH. The 5 windings on the secondary side will
influence
the overall inductance of the loop...
Transformer
Everything is said already. Just have a look at the photograph. I have
to admit, there certainly are better
ways to mount the BNC connector...
Tuning Capacitor
We are going QRP with this aerial anyhow. I found this one in a old
BC-RX (my favourite source for these things). It helps if the capacitor
has two identical sections. Feeding just the stators will on one hand
leave the
rotator and therefor the chassis w/o RF on the other it double the
voltage
rating. The solder joints should be as solid as possible (remember the
resistance story). I used a 80W iron to solder the loop wire directly
to the stators.
Overall View
Showing pictures, it might be reasonable to show the whole thing. What
I found helped, so there ist a shelf's backside steelcross used
to
give support for the bamboo rods. This will disappear soon... making
place
for some more cable ties. Please excuse the poor light quality on this
one,
was made on a business trip in the middle of the night.
Experience
The tuning range is huge. I observe resonance between
5
and
22MHz. Even though it tunes on the 40m band, I
believe transmitting there might not make that much sense - radiation
efficiency would not be that
great. I used the lower range to listen to broadcast.
First QSO on 20m during this business trip (Germany) resulted in a
report of 579 from Hungary using 2W PSK31 (w/ FT817). At this time the
antenna was hanging on a wardrobe (indoors, as you expect). The room
was located at the ground floor of a steelwork office building.
Some news on my rockloop
I went out for some shopping, and that's the result (well, not the Al-boom
on the floor):
These things can be turned into something useful for the rockloop. Up to
now I had it hanging off stuff like curtain rails etc. This, however,
is not a real solution, in particular since sometimes, it is really difficult
to find a mounting point not too close to walls.
Thus, here's a very very cheap and easy solution. The sunshade stand cost me
less than 4 Euros, the spider web broom was about 4 Euros.
Let's see what the mod is like.
The broom itself can be easily separated from it's telescopic stick. Simply
pull (carfully) on the steel bits. Twisting the stick slightely helps a lot.
The result would be something like this:
The top plastic bit actually does all the trick for us. However, there was
some material connecting both legs on the inside part, that needs to be cut off.
This modified bit perfectly fits my loop wire. Like this:
Next step, insert the broomstick into the sunshade stand. I skipped shooting
a photograph of that respective step...
In the sequence of events, the plastic bit will mysteriously find it's way back
home into the business end of the broomstick. As such:
And the final result, as set up in my garage:
80m Frame Antenna
