Tuesday, June 22, 2010

I use a 40/80 meter vertical antenna (a Butternut HF2V) to work the higher bands with the help of an antenna tuner ...

Q John Stewart, W3CID, asks, “I use a 40/80 meter vertical antenna (a Butternut HF2V) to work the higher bands with the help of an antenna tuner. Ignoring the effects of line losses because of the (presumably) lower SWR at the feed point, would I gain any efficiency by switching to a similarly sized vertical designed for multiband operation? Would the performance of the HF2V on other bands be improved if I added shorter radials cut for the higher frequencies?”

A The crux of the problem with your present system is that you cannot ignore the effect of line losses when you try to use the HF2V on higher frequencies, where it is not resonant. In other words, the SWR on the higher bands is not low. While your antenna tuner in the shack is able to provide a 50-Ω load to your transceiver, there is probably quite a bit of loss in the transmission line between the tuner and the antenna. Remember: The 1:1 SWR you see on your antenna tuner’s meter is only present between the tuner and the radio. The higher SWR between the tuner and the antenna, and the resulting loss in the feed line, remains! In addition, there may be considerable loss in the tuner itself since it may be encountering impedances that are difficult to match efficiently.

Let me illustrate, using a model of a simple quarter-wavelength long vertical for 40 meters. I’m going to assume that the ground plane is perfect, so that we have a baseline from which to compare. At 7.1 MHz the feed-point impedance is the theoretical value of 36 Ω and the SWR at the feed point is 1.39:1 for the 50-Ω line. (I’ll also assume that the feed line consists of 100 feet of RG-213 coax.) The total loss in this coax at 7.1 MHz is 0.566 dB, computed using the program TLA bundled with the 18th edition of The ARRL Antenna Book. The coax loss is essentially the matched-line inherent loss if the cable were working directly into a 50-Ω load. There is very little additional loss due to the small SWR at the load.

Now, this very same vertical at 14.1 MHz would be close to a half wavelength long and the feed-point impedance would be very high. The EZNEC program by W7EL computes it to be 814 + j 119 Ω. At this impedance the SWR on the RG-213 would be an impressive 52:1, and the loss in the cable would now be 7.5 dB! The loss in a typical antenna tuner feeding the input of this 100- foot length of coax would be on the order of an additional 0.35 dB. Feeding 1500 W into the tuner would result in only about 247 W radiated by the antenna! Something is going to get hot, mainly the coax.

At 21.1 MHz the situation would be somewhat better since the 40-meter vertical is three quarter wavelengths long and the feedpoint impedance would be 63.4 –j 58.8 Ω according to EZNEC. This is an SWR of only 2.77:1 and the total loss in 100 feet of RG-8 would amount to only 1.47 dB, according to TLA. A typical tuner would lose only a negligible amount more, again because the impedance to be matched is reasonable. In this scenario, the total power delivered to the antenna for 1500 W input is 1057 W. That’s a lot better than on 20 meters.

Unfortunately, another little problem rears its head at this point. Because the electrical length of the antenna is long at 21.1 MHz, the radiation pattern has developed lobes pointing up in the air. You’re heating the clouds rather than aiming for lower angles that are most useful for DXing on the higher frequencies. This problem will get only worse on 12 and 10 meters where the electrical length is even greater.

So, you can see that having a multiband antenna that is quarterwavelength
resonant in the bands you desire would provide far better performance than trying to force-feed your existing dualband antenna by using an antenna tuner in the shack. Of course, you could move your antenna tuner to the base of your Butternut. In this position most of the loss would be in the tuner only. But unless you installed an automatic tuner at the base of the antenna, it would be very inconvenient to use. (See “One Stealthy Wire” by Steve Ford, WB8IMY, in the October 1998 QST.) And you would still be presented with the problem of energy wasting higher angle lobes due to the electrical length of the antenna.

My recommendation would be to switch to a multiband vertical. Butternut and other manufacturers make such antennas.

From QST February 1999