You can get good performance with a dual band 40 meter and 15 meter combination antenna. Most authorities stated that this was not possible. This goes back to my Novice days in 1959, to an antenna design made from multiple 300 ohm twinlead wires cut to supposed resonance on each band EXCEPT 15 meters. We were told NOT to include a 15 meter element in the multiband dipole system. The article was written by a European ham, and I will provide a reference if I find it in my archives. Until now, I just accepted this as the wisdom from the ancients without challenge.
However, due to removing all my antennas for construction projects on the house, I began to install new antennas recently. I decided to abandon all the multiband dipoles I had for optimized single band dipoles whenever possible. I was pleasantly surprised when the project to separate the dual 20 and 10 meter dipoles into optimum height single band dipoles for each band. After all was done, including a 17 meter inverted V, I needed a good 15 meter dipole. I had been using my 40 meter single band dipole with a tuner for 15 meters. I pondered how nice it would be to directly attach an antenna to the back of my solid state rig, without a tuner, on 15 meters as well.
How did all this tradition about the 15 meter element come about? The ARRL handbooks in the range of 1958 to the late 60s showed a multiband dipole for 80, 40, and 20 meters, the most popular bands. They made a passing reference to 15 meter use on the 40 meter dipole. For the tube radios of that era, it was fine, since they had a Pi network matching circuit that bragged about 50 to 600 ohm match range. Novices did not have access to 20 meters, only 80, 40, 10, and later 15 meters, as a swap for the old 11 meter band, which became CB. The ARRL handbook showed a three band 80, 40, 20 meter design, which was built from separate wires, running at about 30 degrees separation from each other, from a common coax feed line. This arrangement works very well. I have used it. Adding 10 meters to it did not provide a good SWR for the most part.
I have found that the upper bands like 15, 10, and 6 meters are not happy playing from a common feed line with lower band antennas. That is why I did separate optimized dipoles for 20, 10, and 6 in my renewed antenna farm. Each is installed with its own feed line, at the correct height for low angle radiation pattern. The 10 and 20 meters are "stacked" so to speak from a common support rope with the 10 meter dipole lower than the 20 meter dipole, about 10 feet apart.
Using a 40 meter dipole for 15 meters has presented some problems. QST addressed that in the last few months (2017), in their question column. It got me thinking, "What if the ancients were wrong? What if we are not using the new tools we have to solve this problem correctly?" All my life, I have questioned long standing rules, and often found my own way successfully, so it is in my genes.
The first problem is that third harmonic operation of a 40 meter dipole cut for mid band or phone results in 15 meter resonance well above 22 MHz, nearly useless without a tuner to match out the reactance. Even cutting the 40 meter antenna for resonance at the bottom of the CW band or 7.0 MHz, did not result in acceptable 15 meter resonance. This was due to a quirk in the way radio waves work inside wires. One solution was to attach wires or figure 8 shaped loading devices about 11 feet out from the feedpoint (a quarter wave on 15 meters). Presumably this loading hat trick changed the 15 meter resonance frequency without affecting the 40 meter resonance. I found that this might be OK if the antenna was tuned in the lower portion of the band, but did not pull the 15 meter frequency enough if optimized for phone on 40 meters. I did use such an antenna, with an additional trick, for its other deficiency.
The second problem is that use of a 40 meter antenna on its third harmonic, 15 meters, produced a feed resistance of 100 to 150 ohms. This was in addition to the reactance caused by being resonant around 22 MHz or above. Even with all the tweaking, SWR on 15 was high and needed a tuner; worse yet, now the 40 meter SWR was screwed up for phone. There is an elegant way to correct the high impedance of the 3rd harmonic operation. Use a short quarter wave (on 15 meters) length of 75 ohm coax, which transforms the 150 ohms down to 50 ohms. I used such a fix successfully, along with the above hat trick. It came close to working with a tube rig (SB-102) at the time, but was not good enough for solid state rigs.
So the hypothesis I set out to test was: Is it possible to make a 40 and 15 meter combination dipole that provides good SWR on both bands, with independently adjustable resonance points? The answer is a resounding YES. I simply added 15 meter wires, cut to the standard dipole formula, in an 90 degree inverted V fashion underneath a standard flat top 40 meter dipole. I found that moving the 15 meter inverted V legs to 30 degrees angle relative to the 40 meter element (looking down on top of the apparatus) improved it even more. Normally, a regular inverted V antenna has to be shortened a bit from the standard flat top dipole, and this is exactly how it turned out. The calculator tool below has a mode for inverted V, to get the correct length, and a tool for one step pruning of SWR. I present the SWR and impedance curves here, as the results of my experiment:
Here is a calculator tool that includes inverted V antennas, as well as dipoles, verticals and full wave loops:
Now I know you are going to have a question about the gain of my lash up. Supposedly a 3/2 wavelength antenna has some gain over a standard half wave dipole. Now we are back to a standard dipole instead of a 3/2 wavelength antenna. This in fact is the main claim to fame of the G5RV antenna, which is a good DX antenna on 20 meters due to being a 3/2 wave antenna IF ERECTED 35 FEET UP. The other bands are simply a bonus for those who have no room for anything else. If properly installed, some of the upper bands have gain in some direction (but the antenna orientation to populated areas must be observed, using a great circle map). The G5RV in fact has LESS performance on 80 and 40 meters than a standard dipole at comparable 35 foot height. Here is a web site with comprehensive discussion on the G5RV, including the gain figures for a properly installed one at 20 meters:
The claimed G5RV gain on 20 meters (3/2 wavelength) is 7.5 dBi or relative to isotropic; I have seen slightly higher figures. The predicted "gain" of a dipole at similar height over ground (not free space) is around 8 dBi. (Do not mistakenly use the figure of 2 dBi for a dipole, which is a dipole in free space). Anyway, the difference is less than an S unit between the 3/2 wave and a standard flat top dipole. W8JI notes in his tests that signal reports were comparable; I trust his carefully gathered data. See:
I did note that the 3/2 wave configuration "heard" slightly better than my ground mounted quarter wave vertical. You often hear that a high dipole outperforms a ground mounted quarter wave vertical. The vertical had half a mile total wire installed under it for a ground. I recently had a number of dead trees removed from my back lot. The trunks had to be dragged out with a winch or a tractor; this damaged the ground system. I just do not have the energy or will to reinstall another extensive ground system at my age. I have since removed the vertical, since my individual optimized dipoles at good height worked better under any of the test conditions I devised, except for 80 meters DX only. If I had a tower of sufficient height (125 feet or half wave), I would still prefer a 80 meter dipole over the old vertical, due to the hassle of all the ground radials and bandwidth problems of a shortened vertical. Anyway, I no longer plan to use 80 meters for that purpose. This article is about a well installed 40 meter dipole, and associated use on 15 meters. The goal was to obtain comparable performance without the use of a tuner. The lack of any significant gain difference between the 15 meter inverted V and the 3/2 wavelength use of the 40 meter antenna on 15 meters did not justify the complexities of the tuner or the hat trick and quarter wave match section.
The results of this experiment produced a tunerless, simple to use, effective dipole system for 40 and 15 meters that allowed INDEPENDENT adjustments for resonance on both bands. Those were the design criteria. I also had the issue of running out of support trees.
There is only one other configuration for a 40 meter dipole that will provide superior 3/2 wave performance on 15 meters. It is actually a Bobtail array. Here it is:
Your mileage may vary. Experiment with it yourself and see how you like it. But now you know the older over generalized information out there that a standard 15 meter dipole cannot be combined effectively with a standard 40 meter dipole is not correct. It was based on trying to do too much with a single coax feed on the other bands.
The Jetstream multiband dipole and some of the Alpha Delta multiband dipoles are a good choice for a beginner who only can put up one antenna, but they need the various elements separated much more than the stock spacers, as shown in my other multiband article. I strongly recommend separate dipoles optimized for each band above 40 meters. They do not take up that much space compared to the 80 and 40 meter dipoles, which are a good combination. The triband arrangement 20-15-10 can be hung vertically over each other on the same tree, with the 20 meter dipole highest, but feed with separate coax feed lines. The older information for sure was correct about not combining 20, 15, and 10 meter bands on one feed line. The reactive effect of the 15 meter dipole ruins the performance of the 20 and 10 meter dipoles. That was why the ancestors told you not to put a 15 meter dipole on the same coax with them. But a 40-15 combo works well for me.