UPDATE (APRIL 2012) - HAMS NOW USING THIS AS 9-BAND ANTENNA
Since this site launched in 2009 I have received several emails from Hams who have built this antenna and are using it on 9-bands (2m and 6m as additional bands without an antenna tuner). The highest SWR I have recorded on 2m is 1.5 and on 6m 1.9 I have been able to key-up 6 local 2m repeaters using 1 watt.
Semi Vertical Trap Antenna
In May of 1974 I received my amateur radio licence. It took little time for me to construct a dipole antenna on my very large lot so that I could talk to the world. In November of that same year my wife gave birth to twin boys. We now had four sons and my amateur radio career was on hold.
Thirty years later I had time for amateur radio but was living on a small lot with restrictions. An article by David Conn in the March / April 2005 issue of The Canadian Amateur reignited my interest in returning to the airwaves using a vertical antenna. After building this antenna I was back on the air but still searching for that elusive antenna that would allow me to work more bands using a simple structure with minimum components.
Several months after building David Conn's antenna a fellow amateur sent me a very interesting article that appeared in the March 1968 issue of Radio Communications entitled Which Aerial?www.yumpu.com/en/document/view/11393432/which-aerial-p-radio-amateurs-of-canada This article was prepared by the Medway Amateur Receiving and Transmitting Society and details their testing of various wire antennas. Their results indicated that the best all round wire aerial tested for multiband operation was a semi-vertical trap. Their test included 80, 40, 20, 15 and 10m bands.
It became a very low-cost endeavour to construct a semi-vertical trap antenna because most of the aluminum tubing used to construct David Conn's antenna could be reused for this antenna. The semi-vertical trap antenna provided very good results except for 20m. Realizing that my knowledge of antennas was very limited I decided to experiment using nothing but what I thought was good logic. I attached a length of insulated wire representing a ¼ wavelength for 20m to the side of my main antenna using stand-offs and trimmed the wire for the best SWR. This approach worked so well that I added wires for 12 and 17m and obtained the same good results with no interaction on the other bands.
Figure 1 provides dimensional information and material sizes for the antenna. The 6061 aircraft aluminum tubing can be purchased from Metal Supermarkets. The length of each section of aluminum tubing is not critical provided they add up to the total length required. If you wish to only have two sections that should work just as well provided the overall length is correct. It is important to add approximately 10" to the length of the intermediate and top sections so that they can be nested and fastened to the section below. My intermediate and top section measure 10'-0" and 5'-0" respectively. The cut length of these two sections is 10'-10" and 5'-10" with the additional 10" inserted into the section below to provide stability.
The completed antenna can be seen in Figure 2 fastened to the side of a wooden / plastic deck. If you visit the Medway site you will note that the vertical section of their antenna is listed as 32'-6" high with a 7.00 MHz trap mounted at the top followed by 27’-6” of horizontal wire for 80m. I communicated with one of their current members who had built the antenna but reduced the overall height to 26'-0". The remaining 6'-6" of antenna height was obtained by using a solid horizontal wire which then attaches to the 7.00 MHz trap. I constructed my antenna in a similar manner stabilizing it with a three point guy system in which the horizontal wire section of the antenna serves as one of the guy supports.
Figure 3 illustrates the base of the antenna which is sitting on an empty/corked wine bottle. A notch was cut into the aluminum tubing where it sits on top of the wine bottle to ensure that any condensation that collects inside the aluminum tubing can drain properly. The top of a 10 foot galvanized ground rod is visible on the left side of this photo to which 6-25' and 6-50' radials are attached. Three of the five stand-offs that support the individual wires for 12, 17 and 20m can also be seen in this photo and will be discussed later. The bottom section of aluminum tubing is encased in a 10 foot length of central-vacuum PVC duct to provide a measure of protection especially for children standing on the deck. An enlarged view of the antenna base is presented in Figure 4.
The top of the base section of aluminum tubing can be seen in Figure 5 where it is clamped to the bottom of the intermediate section. A friction saw was used to notch the outer tubing after which conductive grease was applied to the connection point. Gear clamps were then used to lock the two sections together
An enlarged view of the stand-off which is made from a 6" length of ¾" electrical PVC conduit is illustrated in Figure 6. The outer diameter of the central-vacuum PVC duct is approximately 2". A 2" circular saw similar to the ones used for installing lock sets in doors was used to cut the required radius on the end of the PVC conduit so that it could be fastened tight to the side of the antenna. Slots were cut into the PVC conduit to facilitate the installation of a gear clamp. Stainless steel springs and washers available from TSC were used to keep the wires taut.
In Figure 7 a furnace humidifier motor was attached to a 140 pf tuning capacitor that can be rotated continuously. The capacitor was connected between the base of the antenna and the ground rod. Power for the humidifier motor came from my ham shack. I set my transceiver for the centre frequency of each band and transmitted a low-power signal while the tuning capacitor was being rotated. When I had achieved the lowest SWR I stopped the motor and went outside to measure the capacitance. There was very little variation in the capacitance required for each of the bands. I compromised using a fixed 25 pf high-voltage capacitor.
The SWR results shown in Figure 8 have been confirmed using several different meters. My transceiver is a Kenwood TS 850S (100 watt) with built-in tuner. I operate with the tuner disabled because there really is no need to use it. (I continue to be amazed by the excellent signal reports I have received from around the world and will adapt the antenna for 160m in the near future.
The total cost for this antenna using all new parts should be less than $100. The antenna has experienced several severe ice storms and winds of 90 km without damage.
This article would not have been written without the encouragement and support of David Conn VE3KL who modeled the antenna using EZNEC.
Bob Rice VE3HKY
Foot Note: Testing has indicated that the base of the antenna must be at least 12" above ground for ideal results!
Download figure 8 as a PDF.