Thursday, January 22, 2009

Coaxial Cable 40m Tunable Magnetic Loop Antenna Adding 20 metres

There have been a number of requests to reprint the earlier articles on loop construction by Peter G4FYY in online format. This is the second article describing the modification to support other bands.

Now over to Peter for the description of his loop design modification.


Photograph 1: The Modified Coaxial Cable Loop Antenna with Band Switching

Background
This simple magnetic loop antenna is constructed from Coaxial Cable and does not require a tuning capacitor. The single band prototype for 40m was first described by the author in July 2008 and was subsequently the subject of a successful CARC Buildathon evening when about 10 antennas were constructed by Club members. Several of those who made the antenna have since expressed surprise at the results that can be achieved using only modest transmitter power at 7MHz . This has inspired further development.

Two major modifications are described:-

1. the whole antenna is now rotated 180 degrees so that the small coupling loop is at the base and the 'gap' is at the top as shown,

and

2. a coaxial stub has been added to give dual band operation

Photograph 2: The Band Change Switch.

1. Rotating the Antenna by 180 Degrees.

The original loop was constructed up-side-down compared to traditional designs due to a historical 'hang-over' by the author. In earlier loops, it was more convenient to place the large tuning capacitor on the antenna base rather than at the top of the vertical support. As there is no tuning capacitor in this design, conventional wisdom says to put the high voltage 'gap' in the outer loop furthest from ground (and surrounding objects) to minimise detuning effects. Having the smaller coupling loop at the base also allows the coaxial feeder to be led away below the enclosed area of the main loop.

2. Adding Dual Band Operation
The trick here is to be able to significantly reduce the fixed capacitance of the coaxial cable that tunes the loop to resonance. In the case of a conventional transmitting loop that uses an external variable capacitor, the capacitance is simply reduced by opening the vanes to tune to a higher frequency. In this case, the 'tuning capacitor' is fixed (except for the small adjustment provided by sliding the braid). The trick is how to reduce the overall capacitance sufficiently to cover a higher frequency band while keeping the loop dimensions and the self capacitance of the cable the same. The solution is to insert additional capacitance in series with the outer loop so that it appears in series with the cable capacitance. The effect of combining two capacitors in series is to reduce the overall capacitance according to the formula

Ct = C1.C2 / (C1+C2)

and so breaking the connection in the outer loop and connecting a capacitor in series at this point will raise the resonant frequency.

A physical capacitor could be used but in this is instance, we choose to use the self capacitance of another length of RG11. Then by adding a switch to either short or open this additional 'stub', the loop can be made to operate on either of two bands. The 14MHz stub is quite short and may be run down the back of the vertical support as shown in photograph 3. The connections are shown in photograph 4.

Photograph 3. The additional tuning stub is secured to the back of the vertical support.


Photograph 4: Showing how the additional tuning stub is connected. (The switch shorts out the inner/outer of the stub on 7MHz)

The technique works well. It is found that the size of the coupling loop (at 1/5 the diameter of the main loop) is optimum at both 7MHz and 14MHz and that its position does not need to be changed. So switching bands is a breeze - simply throw the switch.

On 14MHz, the antenna is tuned by adjusting the exposed 'tuning' braid at the end of the stub and fine tuning is by sliding the braid on the outer loop. In the author's prototype, it was found that the braid on the outer loop gave about 100kHz of tuning adjustment on 14MHz, (interestingly, the same tuning range as on 7MHz). The 14MHz tuning stub has no effect when the loop is switched to 7MHz because the stub is short circuited by the switch on this band.

3. Adding other Bands
Further bands may be added to make the loop a truly multi-band antenna. The method of switching in or out the various tuning stubs is left to the ingenuity of the experimenter. The simplest approach would be to use a crocodile clip to connect to one of several tuning stubs running in parallel down the back of the vertical support.

4. Tuning Stub Dimensions
The last column in table 1 gives the calculated lengths of tuning stubs for all of the HF amateur bands above 7MHz. Only the 14MHz stub has been tried where the calculated length appears to be somewhat generous. The actual length of stub found by cut 'n try for 14.2MHz was 77cm with 17cm of braid exposed for tuning and pulled back by about 7cm to tune 14.2MHz. The whole 20m band is tuned by moving the braid at the end of the stub.

Table 1: Calculated lengths of tuning stubs for each Amateur Band above 7MHz, (columns 2 & 6)
Tweaking will be required!

5. On The Air
Due to its electrically greater size on 14MHz, the loop should give excellent results. During setting up, I called LY1CX (Lithuania) on 14.19MHz at 10:25 GMT and received a 5-5 report with 5-7 sent. Not bad for 10W in a wet shed!

References
1. “A Coaxial Cable Magnetic Loop for 7MHz” - CARC Buildathon 23 July 2008


1 comment:

slfhaselwfeuiowr said...

I'm impressed: small multiband antenna that is easy to construct! I was wondering, how well would milliwatt QRP work with this kind of antenna - say, a Pixie or a Rock-Mite?

I can't test this myself yet as I'm studying to get my ticket again, but I'm very interested in small, portable, QRP operation. This antenna looks like it could be one component of such a setup.