Practical_Antenna_Handbook_0071639586

C h a p t e r 2 7 : T e s t i n g a n d T r o u b l e s h o o t i n g 621
Shield
50- noninductive resistor
J 1
SO-239
RF input
J 2
BNC
sampling
100
Brass tubing or
2–3 turn loop of
no. 22 wire
Figure 27.26B HF dummy load with pickup loop.
utes. If longer times or higher powers are anticipated, forced air cooling can be applied
by adding a blower to one end of the cage. The device has been modified by adding a
BNC sampling jack connected internally to either a two-Âturn loop, made of #22 insulated
hookup wire or brass rods that are positioned alongside the resistor element.
Thus, it will pick up a sample of the signal so that it can be viewed on an oscilloscope
or used as a signal source for other instrumentation.
For intermittent key-Âdown periods at the United States amateur radio service legal
limit, the CanTenna—originally sold by the Heath Company, now by MFJ—is an inexpensive
solution to the dummy load requirement. The internal resistor is cooled with
mineral oil. Transformer oils, which historically have contained PCBs, should never be
used.
In recent years high-Âend dummy loads with additional monitoring and metering
capabilities have been introduced by a few manufacturers. The Alpha 2100 Dummy Load
sold by RF Concepts can continuously dissipate a full 1500 W on any frequency from 1.8
to 100 MHz indefinitely! It includes full metering, variable-Âspeed cooling fans, and a
price tag comparable to that of a good used legal limit amplifer!
Documenting Antenna System Measurements
Regardless of the instrumentation that may be available, there are certain practices that
the owner or installer of antenna systems should follow throughout the life of the system,
starting on day one. Here are a few that can save hours and hours of time later on:
• Antenna and transmission line measurements should be made and recorded
when the antenna system is first installed and periodically thereafter. If different
results are obtained at some time in the future, the same measurements should