Model A

On 2 Meters:

18 Elements i-folded Dipole Plus Specia!

Phasing Stub 13 Element Cfllinear Reflector 4-3 Element Colinear Directors

SMSSi

On 6 Meters:

Full 4 Elements 1 FofdedOipole ! Reflector 2-Directors

Stacking Kit

Byron Weaver W82HAL 87 Cottage Place, Apt. 22 Long Branch, New Jersey

The Second Requirement

Get maximum power from your link coupled transmitter,

Although the pi-network is the most popular output coupling circuit, there are si ill many who use untuned inductive coupling to transfer power from their transmitter to the antenna. I his type of coupling is probably most widely used in portable or mobile equipment where the power is low. It appears to be a simple, inexpensive, and space-saving method but the question arises, Are you really getting the most from it?" Chances are, you are not!

Recently, I built a battery operated low power transmitter and decided to use this type öf coupling.

Fig. 1 illustrates a typical circuit, where R represents the load resistance. Assuming that the antenna and transmission line have been properly matched (in which case R is equal to the line impedance), maximum transfer of power will occur if:

1. The tank circuit (CI and LI) has a loaded Q of 10 or less,

2* The pick-up or link coil (L2) has a reactance equal to R at the operating frequency, and

3. Coupling is very tight between coils. Little difficulty arises in meeting the first and third requirements, but if your transmitter fulfills the second requirement, either you are very lucky, or you have taken time to make the worthwhile adjustments!

The object here is to enable you to meet the second requirement! Theoretically, it is possible to compute the approximate inductance, but the approximate is not nearly enough! Mösl articles slate a certain number of turns for L2. This ma\ have been optimum for the author but most likely you will not have a replica of his transmitter, Then too, he may not have adjusted his link properly either.

Trial and error is the method I have found to work most satisfactorily to meet the second

+ HV

Fig, 1. Typical rf output amplifier showing the output coil, LI, the link, L2, and the load resistor, R.

requirement. It requires only time and patience. Once finished, you know the status of your power.

Following is the procedure 1 used to increase my output voltage from 0,15 volts peak-to-peak to 13.5 volts peak-to-peak across a 50 ohm load- The dB meter on the receiver went from 35 dB to well past the 100 dB mark!

Since my coaxial cable was about 50 ohms, I connected a 50 ohm, 2 watt resistor directly across the transmitter output connector, keeping the resistor leads as short as possible. The resistor was the composition type and not of the inductive variety, Connecting an oscilloscope across the resistor 1 found die voltage to be 0.15 volts peak-to-peak with 3 turns on the link of the 40 meter transmitter. The turns in both LI and L2 were close wound on a 1 inch diameter form. By spreading I he turns on L2, the voltage increased to about 3 volts.

his indicated that I had lowered the inductance and/or got closer coupling since there was now no space separating LI and L2. By taking a turn off the link, the voltage increased to about 6 volts for 2 tight wound turns. Opening the turns decreased the voltage, Moving the link closer to, and away from Li varied the voltage tremendously, although moved only an eighth of an inch or less at a time* Finally, a point was wound where I had a maximum of 13.5 volts peak-to-peak for my little rig and I applied a little cement jilue on the pick-up to hold it in place. The results of the adjustment were obvious on the receiver s meter when the rig was reconnected U> the antenna line for it pegged!

Many amateurs don't have a scope available as I do, but can possibly find a friend who has one. Pilot lamps of the proper impedance could be used in place of the resistor by observing the brilliance and adjusting for maximum. An AC voltmeter suitable for the frequency might be a substitute for the scope, eve nil resistors can be paralleled to obtain the wattage and resistance required for your rig. Make sure the leads are as short as physically possible in any case.

After all, if you have given time to build a rig, you shouldn't short-change yourself by-keeping the power in the rig rather than the antenna, unless you are in need of a heater rather than a contact! . . . WB2HAL

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