Q I have a dipole cut for 20-meters fed with 45 feet of coax
and I’m trying to use it on 40-meters with an antenna
tuner. However, I need the full amount of tuner capacitance to get the SWR to 1:1. Does that mean that I am consuming a lot of power in the tuner?
A How much 40-meter power gets to your 20-meter dipole
depends upon both the loss in the tuner and the loss in the
feed line. According to the EZNEC antenna-modeling software, if your dipole is 35 feet above average ground, the feedpoint impedance at 7 MHz is about 13.6 – j1000Ω.
While that doesn’t tell you what you get at the tuner end of
the line, N6BV’s TL and TLW programs (from the ARRL Antenna Book disk) will. If you are using RG-213 coax, the shack-end impedance will be 546 + j806 Ω (SWR 35:1). The same programs also give you the total feed line loss for a given SWR. In this case, the line loss is 16 dB. Clearly, the loss in the tuner is not all you need to worry about!
If you substitute 450-Ω ladderline instead, the shack-end impedance is 14 + j667 Ω (SWR 115:1), but the feed line loss drops to 3 dB. Why is it lower than the coax if the SWR is higher? The answer is that the additional loss due to the SWR is proportional to the line’s characteristic loss, and ladderline has much less loss than coax. A 2.7-dB loss is half of your power, though. If you operate on 40 meters a lot you might want to consider a longer antenna (an 80-meter dipole on 40 meters gives a total feed line loss of less than 0.5 dB with 45 feet of ladderline).
Concerning the loss in the tuner, every tuner design will have a certain amount of loss. Some tuners are more lossy than others.
From QST January 2001
