I am starting to think about my options for antennas for Field Day 2009. I will be
camping @ Monson Lake State Park, MN. I will be operating QRP in class 1B1B. (K2 +
battery @ 5w) I would like to have 2 antennas that will cover 20/40/80 and fit
within a modest size campsite.
There are lots of simple options for 20/40 with wire dipoles. 80m becomes a challenge
because of the size of the antenna @ a 1/4 wave length. I will need to try to check
out the campsite ahead of time to see if a 1/4 wave dipole is an option. Assuming
that I need to go with a plan option for 80m I have an idea for that as well.
20m/40m Dipole Idea
KD1JV inspired 20/40 antenna with some tweaks.
Center to Trap = 16.363'
Trap to End = 13'
Trap is a T50-2 with 24 turns or 2.8uH + a 47pF cap across the core.
A T50-6 with 26 turns or a FT50-61 with 6 turns would also work.
Tuned for 7.05 mhz and 14.05 mhz @ 50 ohm
The 20/40 antenna is modeled @ 30' of elevation. The SWR will drop a bit more as the
elevation increases. 30'-40' is a pretty good comprise height for this antenna.
80m Antenna Idea
80m inverted L with an elevated radial @ 75 ohm
The base is elevated 6' above the ground.
The elevated radial is 10' long.
The vertical section of the antenna is 30' tall (top of the vertical section is @
The top hat wire is 37.5 feet long.
The natural feed point is SWR 1713:1 or Zr/Zi = 20.3, -j1613
Given the ugly feed point values there are some easy options to get a match.
I have chosen two different L networks options because of their ease of construction
and semi-achievable parts. (Almost anything is possible in a model... but
in the real world Mr. Murphy general has some harsh reality checks for us.)
Option 1 for 75 ohm coax
C-L Lo Pass L-Net
Shunt: 782.43 pF, Series: 73.96 uH, Q=225
73.96 uH = 13 turns on a FT50-43
Option 2 for 75 ohm coax
C-L Hi Pass L-Net
Series: 18.2 pF, Shunt: 43.73 uH, Q=225
43.73uH = 10 turns on a FT50-43
There is a lot of room for improvement in the 80m model. For 30 minutes of modeling
it seemed like a decent start.
I happen to have some toroid cores, some variable capacitors, and some door knob capacitors
so these are all values with in reach of my work bench collection of parts. In case
you are wondering why some of the models have a 75 ohm feed point... 75 ohm coax is
cheap. We are talking about pennies per foot for RG6 which I happen to have a few
hundred of feet on a spool near by.
OK... in reality the 80m inverted L will suck. It
is too narrow and tweaky. It was a fun exercise... but I need something more realistic.
(An 80m dipole takes a LOT of space. Space & efficiency is what I am
competing against. I need to visit my campsite ahead of time to see how much
space I have to work with.)