This article may be useful for those who want to experiment or those who can't post full size antenna.
In everyday work in the air I use a small whip antenna. It is a frame with perimeter 4.7 m, made of copper wire (see figure).
At the point the voltage antinodes antenna electrically longer using the variable capacitor (such way electric antenna extension allows to increase the efficiency more than other methods). The power supplied to the antenna from the middle of the vertical side cable RK-50 (tested and RK-75, antenna works not worse than with cable 50 Ohms) through a simple balun device. A few turns of the cable wound on a ferrite ring of the magnetic core, about where you connected the cable to the antenna.
On my radio this antenna is located on the inside of a window frame. It is attached to the frame with tape. Window frame with the antenna is on the fourth floor of a five storey building, the window faces North-East, brick concrete. On this antenna, I worked as a power of 30 W at 14 MHz band, power 10 watts on the bands 21 and 28 MHz. On the "twenty" in a short period this time in connection with 36 countries: CIS, Europe, Asia, Canada and the USA. Connection conducted CW - 50%, SSB - 40% of the ties and a little RTTY and SSTV. Well in CW answered Asia, held about 30 ties with JA From DX - R1MVI, 9M2, YB, 4S.
KPE is desirable to use with a large gap between the plates, as it is at the point antinodes of the voltage (I have used KPE from household tube radio capacity 12/495 pF). To complete the physical length of a cloth frame shall be increased by the length of the wires connecting the frame with KPE. In my antenna these conductors have a length of half the vertical side of the frame, and they made of two-wire line type "noodles". The antenna tuning is done with changes in the capacitance of KPE for minimum SWR, and the selection length conductors from the frame to KPE.
Author: A. Sterlikov (RA9SUS)