CB Whip vs. 10M Double Bazooka: An Antenna Testing Pilot Experiment Using WSPR

OBJECTIVE: Develop a pilot experiment to determine if Weak Signal Propagation Reporter (WSPR) can be used by amateur radio operators to test radiofrequency propagation, and in particular compare antenna systems.

DESIGN: Erect two 10M antennas in close proximity to one another. One antenna is a citizen’s band (i.e. CB) 11M steel whip on the roof of an American work truck (i.e. cargo van). The other is a 10M “double bazooka” dipole hung vertically in a tree by one of its end radiatiors. Both antennas are commercially produced. A standalone WSPR transmitter (by Zacktek, Desktop 80-to-10 model) will produce 200mW WSPR transmissions on the 10M amateur radio band specifically around 28.126 MHz) for two minutes and then rest for two minutes to complete one WSPR cycle. Each antenna will transmit two WSPR cycles. The antennas will be tested back-to-back with loss of the equivalent of one WSPR cycle while switching between them (i.e., 4 minutes).

The CB whip is shown below positioned on the van. The antenna is resonant on 12M, with a high SWR on 10M ranging from 5 to 10 across the band.

The 10M double bazooka is fully suspended vertically in a tree. The feedline is leaving the centerpiece. The SWR is between 1.5 and 3 across the entire 10M band.

The proximity between the two antennas is shown below. The work truck is parallel-parked on a busy street near the Boston Harbor in South Boston, MA. The double bazooka antenna is in the tree marked with an asterisk on the right hand side of the photograph, whereas the CB whip location is marked on the left hand side. The distance between the two is less than 20 feet.

The WSPR unit operating from the floor of the work truck (shown in the header photo) is attached with a microUSB-to-USBA cable to a 12V to 5V converter to a LiFePO4 battery. The GPS antenna of the WSPR unit is positioned outside of the truck. An SMA to UHF pigtail attaches the WSPR unit to either the coaxial cable of the magmount base (approximately 18 feet) or to the 25′ coaxial feedline for the double bazooka.

Two WSPR cycles at 17:18 and 17:22 UTC are sent through the double bazooka (Dbl) antenna. A single WSPR cycle lapses, and the then two WSPR cycles at 1:28pm and 1:32pm are sent through the CB whip antenna.

RESULTS: KM1NDY (the amateur radio call sign responsible for these WSPR transmissions) spots are reported 6 times by 3 stations when using the CB whip for transmission. Likewise, three stations heard the KM1NDY WSPR transmissions from the double bazooka antenna.

Visually, the propagation pattern between the two antennas was extremely similar, despite the very small number of receiving stations.

The following chart shows the call signs of the WSPR transmitters that were detected by receivers during the indicated time. Of note, the striking increase in received signals at 1:18pm ET compared to those detected at the other times is probably due to use of the recommended WSPR coordinated schedule. Please also note that W1BW’s spot of KM1NDY wspr transmission at either 1:28PM or 1:32PM ET is left off this chart; it is unclear why it did not show up on the WSPRNet.org database in this particular query.

Discussion: This was designed simply as a pilot study to determine if experimentation with antenna propagation using WSPR would be achievable. This design accounts for many different variables that could influence results. Antenna placement was very similar, with limited distance between the two. Antenna orientation (vertical) was the same. WSPR transmission occurred in rapid succession when the antennas were switched, thus attempting to minimize the ever changing effects of the ionosphere.

There are several ways to improve this experimental design. Using two WSPR transmitters for simultaneous transmission may work better than alternating transmission cycles, however the physical distance between the antennas would have to be increased. Using a more popular weak signal mode, such as FT8 may produce more interpretiible data.

Ways to compare the test antennas would be to look at number of spots detected, distance of traveled signal, direction of traveled signal, and signal-to-noise ratio

Summary: As demonstrated in this pilot study, a portable WSPR unit can be used to conduct experiments comparing antennas.

KM1NDY