Due to their very nature, HF mobile antennas must be physically short compared to their wavelength. In order for them to hear and radiate effectively, they must also be properly matched to the 50 ohm antenna port on your HF rig.
The impedance found at the feed point of HF mobile antennas varies all over the place. The reason for this is the variance in the mobile environment itself. The size and type of vehicle, the ground area it covers and even the space between the body and the Earth (ground) all have an affect on impedance. You may find that the best impedance match you can achieve results in is a 3:1 SWR at resonance!
Many mobile HF antennas are naturally between 10 ohms and 36 ohms at resonance. With 10 ohms at a 50 ohm feed point the resulting SWR is 5:1! Likewise, 36 ohms yields a 1.4:1 SWR. Matching at 1.4:1 isn’t too bad, but 5:1 is definitely an issue. Fortunately, there is a convenient solution to the problem.
DX Engineering MM-1 Dual Impedance Antenna Transformers are ideal for adjusting the impedance mismatch in your HF mobile! When the natural resonant impedance of the antenna is at 30 ohms or below, the MM-1 mobile unun can provide the needed match. With a 50 ohm input, it provides a 25 ohm port and a 12.5 ohm port to facilitate mobile HF matching.
Let’s say that your antenna analyzer indicates an antenna impedance of 22 ohms. Your SWR without an MM-1 would be 50/22 or 2.27:1. That’s enough to cause considerable power rollback in most modern HF rigs. Using the MM-1 25 ohm port, your SWR would be 25/22, or 1.14:1. That’s a very significant improvement!
Many hams are using antenna analyzers to adjust mobile HF antennas. When a mismatch is indicated at resonance, a DX Engineering MM-1 unun can solve the problem!