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Indoor Loop

Indoor Loop

A loop whose circumference is calibrated to reach the highest selected frequency, 28.300 MHz, while allowing to cover the 21 MHz with a correct efficiency. This allows to use a vacuum variable capacitor (VVC) of only 3-50 pF and thus to get the complete antenna with a budget of about $150.

With a diameter of approximately 1.20m for a circumference of 3.70m, this loop is made with 16mm diameter copper tube. The VVC is connected to the loop via two copper strips 1mm thick and 25mm wide, welded to the tube. The whole weighs a little over 3 kg.


The chosen source is a T-match attacked symmetrically via a 1:1 balun.

Bande (MHz)SWR
28.3 (Fmax) 1.1
24 1.0
21 1.0
18 1.5
Indoor Loop
Band (MHz)BW -3dB (kHz)Q
28 230 (130 kHz at SWR 1.5) 121
21 137 (80 kHz at SWR 1.5) 153


Some photos of the construction, including the solution adopted for mounting the VVC.


This loop is mounted vertically on a PVC tube itself engaged on a portion of PVC fixed on a tripod. It can therefore rotate easily. The whole is placed on a covered terrace located on the second floor.


Rotating the variable capacitor remotely poses some questions :

  • Which motor is suitable.
  • What kind of power supply.
  • How to connect the motor to its power supply.
  • How to control remotely this power supply.
Indoor Loop
Indoor Loop

After some research, I finally opted for the most economical solution. It consists of having a variable PWM power supply in the station that can be inverted -module 1203BB-.

Its output voltage passes through the coaxial antenna connected via two Bias-T that I describe in a separate article.

The wire connected to the Bias-T antenna side is introduced into the tube of the loop at its base and go out at the motor level, a model 12V 30RPM. This is the configuration that provides the least parasitic capacitance and therefore has the least impact on Fo.

The motor/VC connection is formed of a portion of fiberglass tube, in which I inserted and glued a button so that its metal part provided with a clamping screw can receive the axis of the motor. The other side fits on the axis of the VC which is itself provided with a transverse hole and a bolt is used as a pin to maintain the whole.


Indoor Loop
Indoor Loop
Indoor Loop


The vacuum variable capacitor is a Russian model. If its axis is screwed to the stop, there is no mechanical consequence. This corresponds to the minimum capacity and therefore to the highest frequency of the antenna. If its axis is unscrewed to the stop, it is the primary part of the axis that unscrews the device to avoid the risk of mechanically damaging the VC. This corresponds to the lowest frequency of the antenna.

Despite the fact that this antenna covers the 14MHz, I intend to use it mainly on 21 and 28MHz. The attainable stop by unscrewing the axis will never be solicited. As for the other stop, which concerns the highest frequency, the test shows that there is no mechanical consequence and that it will in any case be reached only by accident. Since the PWM module has an independent power supply that is intentionally low in current, the motor stops if its axis is blocked.



The PWM module proves to be even more useful than expected. Indeed, when it is in action and the control voltage is sent to the motor, it generates noise that gets along very well on the transceiver, even outside the tuning frequency of the antenna.

It is enough to attenuate this noise for a weak deviation of the s'meter on the desired frequency and to control the VC. When the latter reaches the target frequency, the noise increases suddenly and it is easy to stop the CV on its maximum.

A fine tuning is then achieved by transmitting while monitoring the SWR. The tuning is finally very fast and it's still better than turning the capacitor by hand.


Magnetic or not

Indoor Loop

By definition, it's not a magnetic loop. Indeed, its circumference is too important. On 28.3 MHz (lambda = 10.60m), the 3.70m represent more than 1/3 of lambda while it should be between 1.29 and 2.57 m according to the technical literature.

But it has a vertical polarization whose directivity follows the plane of the loop and the simulation confirms it. Compared to the tests previously carried out by placing in the same place a vertical 1/4 of whole wave with ground plane which proved totally deaf and inefficient, this loop is doing much better both in reception and in emission.


A polygon generator for MMANA

Using MMANA-GAL for the simulation, I was soon confronted with the fact that it is not possible to realize a circle with this software. It must necessarily go through a polygon that quickly becomes tedious to build if you want a number of sides.

So I created a polygon generator that automates this task and so you do not have to worry about calculating the coordinates one by one. You can use it by clicking here. It is provided with a user manual.

You can download the .maa file (for MMANA-GAL) from this loop whose polygon was made with the above generator :

Download the .maa file


80-40m loops of one to three turns

Three loops that I modeled on two bands, 80 and 40 m.

If increasing the number of windings reduces the antenna diameter, the most effective is the one that has the largest area. It is therefore the simple loop of 2.50 m in diameter which is the most efficient and which has been the subject of a realization.

Here are the MMANA (.maa) files, one by band (only the value of the CV changes), for each of these loops which include one to three turns and have a diameter of 2.50m, 2m and 1.20m respectively.




In physics, we often expresses the wavelength with the Greek letter λ (lambda).


MMANA-GAL is an antenna simulation software, allowing to visualize the radiation, to optimize the dimensions according to criteria of gain for example and is probably the easiest to use.

MM HamSoft

S.W.R.Standing wave ratio

When a transmitter is connected to an antenna, it is hoped that all the RF signal will be radiated from the antenna. In practice, some of the RF energy is reflected back to the transmitter. Reflected energy can damage the transmitter components. The ratio of the forward power and reflected power is the S.W.R. value.



Some tools

For the development of these antennas, the ideal is to use an antenna analyzer or a noise bridge. Here are some useful links:

Noise bridges
MFJ202B from MFJ
RX100 from Palomar (no longer available)

Antenna analyzers
VA1RX from Autek (no longer available)
MFJ259 from MFJ or MFJ-223, MFJ-225
miniVNA HF/VHF (on PC).

Tenna dipper
AA 908 from american qrp club