I’d been wanting to get on 17 Meters (18.068-18.168 MHz) for some time, and I saw that as solar activity increased, there was more and more activity on it! So, based on my success with my earlier 80 Meter inverted Vee antenna, which always gets very good signal reports with my 200 watts, I resolved to make a single band 17 meter inverted Vee. I already had a home made wooden mast base I had built for an earlier experimental magnetic loop antenna, specifically sized to hold one of the 2.25 inch diameter 48 inch long Army surplus tent poles I had previously bought 24 of. These tent poles fit inside each other making stacking easy, and they’re cheap, and they are perfect for the antenna experimenter. Here is an example listing on Ebay, where I obtained mine.
This is the mast base I built for the flat roof on the back side of our house. The mast section just slips into the wooden cylinder, and it can turn freely if needed. This was a fun little project in itself.
In designing the antenna wires, I relied on the trusty formula 0.95*468/frequency to give the length of the overall antenna in feet. This gives a length of each arm as 12.9 feet. So I built each arm like this, beginning at the feed point at center:
- 4 feet of 12 AWG solid wire
- 5 feet of 14 AWG solid wire
- 5 feet of 18 AWG solid wire
So, this is a tapered wire antenna. The motivation for this is to reduce weight aloft. I mentioned this in an on-the-air conversation and was asked about it, so I guess this is not conventional, so I’ll explain. It’s easy to visualize that using very thin wire for a transmitting antenna will hurt its performance, due to resistive losses. But, the resistance of the wire is only a factor where current is high, and in this antenna the current is only high near the feed point, at the center. The current is zero at the ends of the wires (but the voltage is high there.) So, it is a waste to use heavy copper at the ends of this antenna. Before you consider this idea for your antenna, though there is this one important fact: this only applies to a single band resonant antenna. If I were planning on using this antenna on both 18 MHz and 54 MHz, this would not work well.
Also, this is a direct fed antenna. The RG58 coax with a characteristic impedance of 51 ohms, is connected directly to the two legs of the antenna without any kind of balun, transformer, or matching section. That means that the antenna must be configured so that its feed impedance at resonance is exactly 51 ohms. At the other end the coaxial cable is connected directly to the radio without any tuner (the radio itself has an autotuner, but the object is to have such a good match that it is unneeded). By eliminating tuners and baluns and other hardware, the losses are minimized and the available watts are put to the best possible use. It’s been my impression that hams sometimes rely on tuners and SWR measurements too much — if you add enough hardware, you can, as they say, match your radio to a doorknob. But that won’t radiate any power anywhere. Performance needs to be measured by radiated power, and reducing losses and increasing antenna efficiency is the way to achieve it if you cannot add raw power to your transmitter. An inverted Vee antenna could be thought of as a drooping dipole. A straight dipole has about a 75 ohm impedance at the center drive point, but as the arms are lowered to make the Vee shape, the impedance falls. For a 51 ohm cable as feed, the object is to adjust the arms so that the drive impedance is exactly 51 ohms, giving an ideal match. In an inverted Vee antenna the drive impedance at resonance depends on two things: the angle between the two legs, and the presence of other conducting objects in the vicinity. With the center point as high as I can get it (about 20 feet above roof level) the effect of other objects is not enough to matter, so the rest of the problem is just the tuning of angles and lengths.
Tuning the antenna consists of two related tasks:  setting the angle between the two legs so that the drive impedance matches the characteristic impedance of the coaxial cable (51 ohms), and  trimming the length of the wires at the ends so that the resonance is at the frequency you expect to transmit the most. If you have 14 feet of wire at both legs, you can fold up one foot of wire on each end, and form a loop there. You do NOT need to cut the wire. Just fold it. Resonance is set by the reach of the wire, not its length. (I would have saved myself a lot of trouble in my first Vee if I’d known this!) Before you start, stretch the wires in a way that results in about a 120 degree angle between them.
- (1) With the Vector Network Analyzer (VNA) at the cable end, where the radio connects, measure the impedance at the frequency of interest. You are aiming for 51 ohms. If you are on target, go to step 3.
- (2) Go back to the antenna. If the impedance was over 51 ohms, reduce the angle between the arms. If lower than 51 ohms, increase the angle. Return to Step 1.
- (3) Measure the resonant frequency with the VNA. Go back to the antenna and either shorten or lengthen the legs, symmetrically. Shorten if low, lengthen if high. Just folding the ends is adequate, no need to cut.
- (4) Back at the VNA check the frequency and the impedance. If your frequency is right on, and the impedance is 51 ohms, you are done. If not, go back to step 1 or 3 and fix whichever parameter is worse.
The legs don’t have to be symmetrical with respect to the ground — in this case I ended up with a nearly horizontal leg and a steeply sloping leg.
The Vector Network Analyzer I use is this super cheap “NanoVNA” from Amazon which does a surprisingly decent job:
Here below is how the antenna looks on my rooftop, as seen from one of the legs. The nearby leg is tied off to a tree, the far leg is tied to a fixture on the rooftop:
Using the Nano-VNA, after about 10 trips up and down the roof, I got this very good result. The Channel 1 drive point signal magnitude measurement shows a strong dip right at 18.2 MHz (yellow) indicating very good resonance, the Smith Chart shows 49.7 ohms there (green) for an excellent match, and the SWR shows a result of 1/1.04 (blue) which means losses will be very low. My radio (FTDX5000) likes this drive condition very much and shows a nearly 1:1 SWR on its display too.
Here is a closer look at the behavior between 17 and 19 MHz.
So how did it work? Within a half hour, this easy-to-build antenna had given me my very first-ever 17 Meter SSB contact, with N3AIN in Hazleton, Pennsylvania, and a 5-5 report on signal strength for my 200 watts. The next day, I contacted RN3CT, a 100-watt Russian station outside of Moscow. I’m looking forward to having lots of fun on this WARC band in the future as the solar cycle progresses. Not only that, but the WARC bands such as 17 and 30 meters are free of the boring contesters that fill up 80M and 40M on certain weekends, making them good spots to hang out if you don’t want to join the noisy competition.