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The normal-mode helix antenna
RSGB VHF/UHF Handbook.
Much has been said for and against what is termed the normal-mode helix as used on hand-held transceivers. Unfortunately the method of operation and the results obtainable for this type of antenna have been much misunderstood by amateurs and professionals alike. Most theoretical papers only consider the helical equivalent of the Lambda/4 whip while most users of this antenna are in fact using the equivalent of a physically reduced 3Lambda/4 whip.
A helix will work in the normal mode when the diameter and pitch of the helix is less than 0.1 Lambda When working in this mode the radiation is from the side of the helix, and when the diameter is considerably less than 0.1? the resultant 'spring' has a radiation pattern similar to a short vertical monopole or whip.
A 3Lambda/4 whip over a moderate ground plane has a resistive match very close to 50 Ohm. If this whip is coiled into a helical spring as previously described it will resonate to approximately 50 Ohm but at a somewhat lower frequency. If the spring is trimmed to the original frequency the result will be an antenna of about 0,1 Lambda long matching to approximately 500. The actual wire length tends to be around U2 to 5^/8 long at the working frequency. The capacitance formed between the turns of the spring has 'loaded' the antenna such that it still resonates as a 3^/4 antenna. This capacitance also tends to modify the matching under various conditions.
Because of its construction, the spring is very reactive off-resonance and this makes it very important that it be resonated for the specific conditions that prevail in its working environment.
Fortunately it is only necessary to change the number of turns to resonate the spring over such diverse conditions as a large ground plane and no ground plane at all. However, the match referred to 50Q can vary between about 30 and 150Q
at the extremities. Under typical hand-held conditions, however, and depending on the frequency of operation, the spring tends to be fairly close to a 50Q impedance match. This is shown in Fig 5.98 which also gives an indication of the number of turns required for a typical 9mm diameter helix for 3'k/4 resonance.
An important consideration is that since the helix is a reduced size and aperture antenna two factors arise. First, the radiation resistance is lower than the equivalent linear whip so the choice of a good conducting material is important to minimise resistive losses. A steel spring compared with a brass or copper-plated helical can waste 3dB of power in heating up the spring. The aperture of the helical is a third the physical size of the U4 whip and would moreover indicate a loss of4.77dB. However, results obtainable with copper-plated, Neoprene-sheathed helical antennas, correctly matched to a hand-held transmitter at 145MHz, are at worst -3dB and at best are +ldB compared to the equivalent X/4 whip (which is -6dB compared to a A/2 dipole). One thing that will be seen however is that the top of the spring on a hand-held transceiver will often need to be raised to a position corresponding to the top of the equivalent X/4 whip to receive or transmit the maximum signal strength.
A similar device resonated on to a 'kl1 square ground plane could give results 2-3dB below a 'kl1 dipole. An alternative arrangement using a bifilar wound helix gives identical results (within 0.2dB) to a Lambda/2 dipole.
The helical or spring has an interesting and difficult operating characteristic when supported close to the body, particularly at the higher frequencies. Fig 5.99 shows the typical results of a 145MHz or high-band spring and a 70MHz or low-band spring as it is brought closer to the body. The interesting effect which occurs at several centimetres from the body can be seen, where the resonance of the spring, instead of continuing to decrease due to body capacitance, suddenly increases the frequency of resonance. At 2cm and closer the operating frequency suddenly decreased due to body capacitance. Unfortunately this very changeable area occurs at the typical mounting distance of a body-worn transceiver. However, many transceivers are required to be raised to the mouth when transmitting and this puts the antenna back to its best operating conditions.
The normal-mode helical antenna can be vehicle or ground-plane mounted if desired. The height is typically less than 0.1 A, and the gain is around to 2-3dB below a dipole. An acceptable match to 50 Ohm can often be achieved by simply trimming the resonant length. Alternatively, a small inductance or capacitor across the base or a shunt feed as shown in Fig 5.100 will provide the required matching.
И.А. Доброхотов (UN7GM),
Республика Казахстан, 480000, г.Алматы,
Главпочтамт, а/я 178. firstname.lastname@example.org