"Stealth DX-ing from your apartment" by PA0FVH

Usage of a large metal object of the flat building as an vertical antenna.

When it is not possible to erect an antenna, because of restrictions (HOA) or simply because there is no space to do so, you could examine the building for large metal objects, which could be used as an HF antenna. The aluminum fire escape of the flat building for instance is such a large object, which could be used as a vertical antenna. How to bring this thing into resonance? An antenna tuner in the shack was for me out of the question. I didn't want the transmission line (ladder line or coax) to radiate between the tuner and the antenna. On a hamfest I bought the CG-3000 remote automatic antenna tuner and started my experiments to bring the ladder into resonance. The counterpoise consists of 2 radials per band, except for 160m, burried between the tiles on my patio. During my first tests I was driving the ladder directly and with success. To increase performance (RF current distribution over the entire ladder, instead of flowing directly to the brick wall) I have attached a wire in parallel to the ladder and connected this wire to the tuner. The wire is hardly visible on the aluminum cage of the ladder and this makes it a stealthy antenna. I am using this antenna over a year now and the results are very promising. On 80, 40 and 30 meters I have made contacts outside of Europe (in PSK and 40Watts output). On 30 thru 10 meters my magnetic loop is performing better for DX, probably due to a lower take off angle.

Spec's:
• Aluminum fire escape - 10 meters long and 1 meter in diameter.
• 2 radials per band 80m - 10m.
• Remote automatic antenna tuner CG-3000.
• Frequency range tuner and antenna 160m - 6m.
• Wire used: half of zip cord 2.5mm2 stranded silver plated wire with transparent isolation (other half is copper with transparent isolation) used as loudspeaker wire.
• RF choke (2x) coax to shack and 12VDC.

Click to enlarge the photo's :

A magnetic loop antenna is easy to construct and is suited for working DX.

The magnetic loop antenna uses the magnetic component of the electromagnetic field and because of this it size is much smaller than the conventional electromagnetic antenna like the dipole. This makes this antenna ideal for apartment and or limited space environments. In contrast with vertical antennas the magloop does not need any radials while its low take of angle can compete with vertical antennas. The bandwidth of the magloop is very small on its resonance frequency which means that you have to tune the antenna a lot more than a conventional antenna. For apartment dwellers I recommend to use the digital modes of operation or CW and not SSB to avoid RFI at your neighbors, which are many and nearby. If you go digital like PSK or RTTY the small bandwidth of the magloop is not a big problem anymore, because the digital modes frequencies are within a 10 to 20 kHz spectrum in our amateur bands. That's why little re-tuning is needed for working with the digital modes.

Tips to construct a magnetic loop antenna:
• Use both Google and the ARRL antenna book to get familiar with magnetic loop antennas.
• Don't try to construct a 160 - 10 meters loop, but focus on 160-80, 40-20 or 20-10 meters.
• Use a magloop calculator, like the one from KI6GD to get information about efficiency, bandwidth, capacitor value and voltage.
• For maximum efficiency, with commercially available components, make the loop as an octagon, use copper pipe with an OD as big as you can get. In DIY stores copper pipe OD 22mm was the maximum I could get.
• The lower leg of the loop should be 50cm plus the diameter of the loop above the ground.
• Find a suited vacuum variable capacitor 5kV for 100Watts PEP transceiver output. Remember that a vacuum C has at least 10 turns for min max tuning; this avoids a huge gearbox on the DC motor to drive the capacitor for tuning. Try to get a vacuum C that suits your frequency range, this will make tuning more accurate than for instance a 1000pF capacitor. A stepper motor with a wide spaced air variable capacitor could be an alternative.
• Use a gamma match to drive the loop, in my opinion more robust than the faraday loop.
• An antenna analyzer is a useful tuning aid, however you could tune the loop on max noise.
• Instead of using a vacuum capacitor you could use a piece of coax cut for the capacitance needed. I have tested my loop is way on 20 meters.

Data of my loop:
- Copper pipe OD 22mm
- Diameter of the loop 118cm
- Vacuum C 4 - 100pF (10 turns) ex. military equipment
- DC motor with 1:150 gearbox
- Frequency range 30 up to 12 meters
- Efficiency 60 to 97%
- Gamma match

I have made contacts with New Zealand, Antarctica and the America's with this antenna from my apartment patio in PSK with 40Watts. After usage of the magnetic loop I store the antenna inside. The last frequency used by the antenna is stored in the antenna analyzer, so next time you use the antenna you know its frequency (if you don't use the analyzer for other measurements in the mean time).

Here's an impression of my magnetic loop antenna: