Source / reference: G3TXW
I wanted to test a simple airband antenna design since I had a roll of balanced feedline laying around in my shack doing nothing useful. First I had to measure the velocity constant of the feedline with my MFJ-259 to be able to come to an estimate of the required length of the matching section. I could also have used my vector network analyzer (VNA) to do that by the way. To measure without interference from coupling to adjacent objects I did the measurement with the cable hanging out from my balcony as you can see in the left picture. The MFJ-259 was connected in the end and held by hand (that is a benefit of the battery operated MFJ 259 even if the instrument is not of the most accurate on the market). I wanted to make two antenna segments folded over each other. Therefore the top of the feedline is shorted and the currents will be in phase if the antenna is of a proper length. The matching section is a shorted line section that is tapped by the transmission line. The coil on the coax is a choke (I haven’t done any measurements on that choke yet by the way).
The procedure I used to find the velocity factor of the balanced transmission line was to first measure a length of the feedline with a tape measure. Then I connected the MFJ-259 and found the frequency where the lowest reactance could be measured. (See pic two from left on the upper row above. You can see that the X is very low). This was done in the mode of the MFJ-259 where it is possible to measure both R and X. This is the quarter wave frequency of the line when the wave propagates in the line – not in the free air (“ether”). Then I calculated that frequency back to the wavelength with y=300/f. I then divided the tape measure length by the calculated length and came to a velocity factor of 0,89. This is the ratio of the wavelength in free air and the wavelength in the transmission line. This is directly related to the propagation speed of the line when it operates in transmission line mode. From that I calculated the required length of the matching transformer and the approximate tapping point on that transformer to reach 50 ohms. Please note that you cannot use the velocity factor of the transmission line to calculate the required length of the antenna (only the matching section), since the RF currents on the two folded legs on the antenna are in phase and therefore the one lead is coupled to the ether and not to the other lead.
The picture above shows the SWR as measured with my vector network analyzer from DG8SAQ. The markers on the right side shows a 1:2 SWR bandwidth of 118,5 to 128,6 Mc/s which is OK. The reference level is 1:1 SWR. This level is lifted one division for clarity. (I think the Mc/s is a cool way to express frequency by the way.)
Here my DG8SAQ1 kc/s to 1,3 Gc/s VNA is shown. It is connected to my PC via USB.
Conclusion: a combination of the MFJ-259, the DG8SAQ vector network analyzer, some balanced line and some coax can be used to make a good collinear airband antenna in less than one our at a cost of a few dollars. The antenna was screwed to a wooden section of my roof by a small screw by the way and can be removed in approx 2 minutes.