Topband: VERTICAL ANTENNA MATCHING

[K9AY]

Bill...

>> ...I have seen very few suggesting using an inductor at the base to 
>> ground, tapped down for resonance and up from ground for 50 ohms.  Same 
>> thing on ON4UN's book-I believe there is one reference to this method of 
>> matching but many using L-networks and ununs.  What is the issue here: 
>> efficiency?

As N6RK noted, your antenna needs to be shorter than resonant (capacitive) 
to make this work. Perhaps the biggest benefit is that once you are set up, 
you can do fine-tuning at ground level if it is a short antenna with narrow 
bandwidth. On the downside, high-Q inductors take some care in construction, 
and will degrade a bit over time as the metal oxidizes.

You can also use a capacitor to ground instead of an inductor. In this case, 
the antenna is physically longer than resonant (inductive). Capacitors have 
higher Q (lower loss) than inductors, and high voltage disc ceramics are 
cheap. Most inv-Ls will match with 1800 - 2200 pF, so use 8 or 10 smaller 
values in parallel to handle the current.

The shunt L or C primarily provides the impedance transformation, so in 
either case things work out easiest if the antenna itself can be pruned to 
the desired frequency. Otherwise you need another component to control the 
frequency and it becomes an L-network. This also works fine, but adds to the 
complexity.

>> The bonus seems to be that such a configuration makes the antenna 
>> somewhat quieter on receive and may offer a measure of protection from 
>> Thor.

'DC' ground will not affect RF noise levels, but the static discharge 
comment is true. For the shunt capacitor match, an RF choke will bleed off 
static.

73, Gary
K9AY