Background

Losses due to hand-held transmitter with a whip antenna are significant. Furthermore, the natural tendency is to point a r/c transmitter at the airplane, an orientation that gives the weakest signal. Replacing the whip with a transverse dipole increases the signal strength by increasing the efficiency of the antenna system and allows pointing the transmitter at the airplane to give the best signal.

The typical r/c transmitter uses a 39" whip antenna, which in terms of wavelengths of the frequency bands 35, 50, and 72 MHz, is 0.11, .17, and .23 wavelengths, respectively. The transmitter is hand-held. Since there is no ground-plane, or radials, the person holding the transmitter is an important part of the transmitting antenna system. The radiation resistance of the whip at these lengths & frequencies is small, but the human body is high resistance. Consequently a significant amount of the transmitted power is absorbed by the person holding the transmitter.

A search turned up one IEEE article applicable to the r/c whip antenna situation [1]. At 57MHz, with a 39 inch, (0.19 wavelength) whip antenna, losses in the 11.4 – 14.5 dB range were measured. At 34.8 MHz with a 39 inch, (0.11 wavelength), antenna the losses ranged 19.1 – 23.7 dB, and at 79.4MHz (0.28 wavelength antenna) the range was 6 – 6.1 dB. The frequencies and wavelengths for the measurements in the report are very close to the r/c frequency bands with a 39" (one meter) antenna, so the measurements should be representative of the r/c situation.

These losses are very large, e.g. for the worst case at 35MHz less than ½% of the power is radiated!

Antenna pattern

When one holds the r/c (airplane) transmitter in the normal position, the natural tendency is to point the transmitter at the airplane. For a whip (and rubber duck) antenna this is the worst position as this points the weakest signal at the plane, i.e. the null is off the end of the whip. A null can also occur due to "cross-polarization," such as when the transmitter whip antenna is vertical and the airplane receiving antenna is horizontal, but this is not usually a problem.  Pointing the antenna at 90 degrees to the plane is not natural with a whip or rubber duck antenna.  A transverse dipole overcomes this problem.

Transverse dipole

A dipole that has the antenna elements "crosswise", or transverse, which improves the antenna system two ways:

1. The losses due to the human body are eliminated. The antenna is self-contained and does not depend on the person holding the transmitter.
2. The best signal is "off-the-side," as is also the case for the whip, however since the antenna is transverse to the transmitter, pointing the transmitter at the airplane puts the best signal towards the airplane. Holding the transmitter vertical (e.g. some people shield the sun by holding transmitter vertically) still leaves the transverse antenna horizontal so the cross-polarization null is also avoided.

Design

All the work was done for the 50MHz band, (channels 01-09). Appropriate scaling of values would be expected to work for 35MHz, but at 72MHz the length of the antenna will be about ½ wavelength and as such a different matching scheme might be better. (Note also that at 72MHz the losses of the whip/human antenna are less than 50MHz so the improvement in signal strength with a transverse dipole will not be as great, however it still will have the advantage of being able to point at the airplane for best signal.)

Two telescoping antennas (Radio Shack 270-1418, approx $6 each) are mounted on a plastic box. These elements extend to 38 ¾" (each side) and inside the box is another 2 ½" on each side to bring the connection to the center. This makes the total length of each leg 41 ¼". At 50.8MHz a ¼ wavelength leg would be about 57". As a result of the less-than-1/4-wave legs, the impedance is a resistance which is lower than a dipole, plus a capacitive reactance that needs to be tuned out. The antenna requires a balanced feed as well.  A toroid, wound bifilar to achieve tight coupling, balances the feed and the trimmer capacitors in conjunction with the toroid inductor tune and match the antenna to the coax.

The impedance of the antenna (50.820 MHz) when modeled with NEC is approximately (28 – j278), or 28 ohms resistive with a capacitive reactance of 278 ohms (which equivalent to about 11.3 pf). (Rohde & Bucher [2], for a 3 foot whip, shows a resistance of about 75 ohms and a capacitance around 12 pf., but this does not take into account the loss affect of the person holding the transmitter.) The 278 ohms reactance and 28 ohms resistance makes the Q of the antenna about 9.9. Since high Q values mean the antenna will be easily detuned by nearby objects I felt it best to lower the Q. To bring the Q down, capacitive loading in the form of end caps from the bottoms of beer/soda cans was added. The measurements were then (36 – j117) for a Q of 3.25. (Note: later, the antenna without end caps was measured to be (34 – j168) for a Q of 4.9, which is much lower than the computer modeling showed, so the end caps may not be as useful as originally expected). However in practice, the end caps were found to be handy for extending/collapsing the antenna as well as being more visible, making it less likely to operate with the antenna not extended.