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Here is a useful project from Steve Weber, KD1JV 
The project / schematic can be found here:

Steve 'Melt Solder' site: KD1JV Designs

Please see all the projects on this great site.
You may purchase the Tenna Dipper kit from QRP KITS,
or homebrew it as I did, with some modifications.

It is used to analyze either:
1) what frequency is at 50 OHMS on your antenna
2) assist in trimming the length your antenna for a given frequency.

It is a Wheatstone bridge where the antenna is one leg. No current will flow past the bridge when the antenna leg is 50 OHMs. This 'dips' the LED.

My modifications include:
1) building in my own homebrew 5 digit frequency counter instead
    of the one designed on the schematic.
2) including a 10 mA meter to watch the needle dip along with the LED dip.

From the Hendricks QRP KITS WEBSITE:
Wide tuning range: ~3 MHz to ~30 MHz in two ranges
Four digit digital display with display shift down to 100Hz
Eliminates the need for a transmitter and SWR bridge when:
Trimming length of resonate dipole, and portable vertical antennas
QRM free adjusting of antenna tuners and antennas
Quickly adjust short, portable vertical antennas, eg. PAC-12 & Screwdriver antennas, etc.
Current draw is approximately 50mA

My Completed Dipper
Tenna Dipper

A) The schematic and boards were layed out using EAGLE CAD 5.11, 
     the board patterns were then laser printed onto acetate.
B) The PCB patterns on the acetate were heat transfered to the copper boards,
     and the boards were then etched with ferric sulfate.
C) The same acetate / heat process transfers the silkscreen to the top of the boards after drilling
D) The led / meter cutouts on the metal case were done with a homebrew CNC milling machine.
     (But thats another story, for another page, for another day.....)

tenna dipper inside