What happens if you fold the ends of a dipole?
Folding is used where a bent dipole needs to be more compact.
For example, if it is found that the ends of a bent dipole touch something or prevent rotation, consider folding. Or, in a Yagi type beam, folded ends would be an interesting possibility to make a smaller antenna. Or, for antennas on 160 and 75/80 meters or 40 meters in an attic, having enough height is the eternal problem. Folding may be something to consider.
To study folded dipole configurations, an antenna model was designed based on: 1) a calculated resonant Length (velocity corrected), 2) a bend Ratio, 3) the vertical Length of the bend. In this way tuning becomes simple. Just lengthen the tips of the folded ends to find the lowest SWR. (See Figure 1)
For the antenna builder this generalized technique makes the results of the study scaleable to any frequency of interest.
4NEC2 Antenna Model: Here
The study involves seting a Ratio to fix the length of the dipole arms, then systematically adjusting the length of the vertical BEND section to find each corresponding Length of the FOLD section by optimizing for "SWR" and "X-in".
As the study progressed it became clear that the bend length cannot be greater than 0.5, that is to say, the dipole cannot be shortened by more than 50%. Similarly, the dipole bend length must be at least 0.8 or no shorter than 20%. The intermediate lengths of 0.6 and 0.7 were also used for the study.
In Figure 2 below is the modeling result of these four lengths of the dipole arms.
The graph presents the optimized SWR results versus the ratio value of the vertical downbend...
(= ratio times the calculated length of the dipole).
For example, on the Green curve the lowest SWR is around 0.06 Ratio.
At 14.2 MHz: 0.06*150/14.2*0.95=0.6 meters for the length of the vertical downbend
In the the Bent end study before this study, the SWR sweet spot was at the 0.6 length. With this Folded end study the 0.7 length (Green curve) is favored. At the point where the SWR is lowest, both sides of the fold are about the same length.
As you can see from the SWR curves above, folding a dipole is very forgiving. Just about anything produces a good match for 50 Ohm coax But, if you use the 0.5 curve (Blue line), it is advisable to use long sides (Ratio > 0.18). In that way the SWR should be under 1.5 at 1/2 wavelength elevation.
A straight dipole is the perfect form. Any bend (or sag) has the effect of shortening the radiating body, hence frequency rises, impedance falls. The way this study is designed, the Arm, Bend and Fold add up to the calculated length of the dipole. Only a little length need be added to the tips of the folds to correct for the shortening effect.
Figure 3 shows what happens to the SWR tuning versus how much length is added to the tips. Again the 0.7 curve stands out. It has the lowest SWR over the broadest range; obviously the easiest to tune. Conversely, the 0.5 and 0.6 curves indicate little posibility for improvement in SWR.
A perfect halfwave dipole is also 1/2 wavelength high. That is a tall order for the low bands. What ham can put up a 180 meter antenna at 267 feet? ... or an 80/75 meter antenna at 130 feet? Reality is more like 30 feet for most hams and perhaps 60-80 feet for a few. This means 20 meters for most and 40 meters for a few. Lower than that, performance goes downhill.
Figure 4 below compares what happens to a folded-end dipole that is 0.7 long when it is mounted near ground, that is, less than 0.1 wavelength high.
At 0.5 WL high the folded end dipole performs well indeed. The DX radiation pattern is perfectly normal. Gain is remarkedly flat around 8 dBi (Green line). The difference in gain between a straight dipole and a folded end dipole is undetectable, less than 0.2 dB.
At 0.1 WL elevation the same antenna radiates vertically in a perfect circle and exhibits what is politely called "unity gain"... more likely some loss. DX radiation vanishes and the Green line drops about 6dB to become the horizontal dashed Red line.
Furthermore, at around 0.1 WL the inescapable fact is most of the RF of any horizontal antenna is reflected skyward by the ground. However, some of skyward signal is reflected back down on 160 and 75/80 meters and often on 40 meters. If you live in a valley or have a mountain range in front of you, or participate in ARES or MARS, this Near Vertical Incident Skywave (NVIS) will help.
Notice also the big change on tuning as the downbend segment is longer (larger ratio). The flat Green line becomes a Dashed red curve. This says "no fold". Very near ground a straight bent end gives the better SWR.
In summary, a folded end dipole:
1) Performs like a straight dipole.
2) Is tolerant of how it is folded.
3) Is simple to tune
4) Can be up to 50% shorter
Dick Reid, KK4OBI at qsl.net