INTRODUCTION:
Last year at the OKC "Ham Holiday" hamfest I picked up a 6m vertical "Hamstick" antenna. The price was right and most folks say they're an OK antenna. Probably nothing to write home about compared to a true 6m quarter-wave whip, but if I really get into 6m, I'll consider it.
Anyway, I have a 10m Hamstick I need to get ready for the 10m nets, so I considered this practice.
Link: http://www.shopjubilee.net/itm00422.htm
WHAT IS IT:
A Hamstick is a physically short (compared to a 1/4-wavelength) antenna consisting of a helically-wound section and a stainless steel whip that must be adjusted up and down via a set screw. The name of the game is to get this whip the right length then cut it off. What length? Well the box said it should do a 1.5 SWR, so that was my goal. And I knew I wanted to operate on the 50.1 SSB area and the 52.525 MHz FM simplex areas.
MATERIALS:
1. a "sharpie marker"
2. pad and paper
3. a scientific calculator (with a square root key)
4. a wattmeter
5. my VX-7R HT set to FM as a signal source.
PROCESS:
After reading the directions, I made some Sharpie marks every .25" from 4" to 5.75" from the end of the whip facing the black core of the antenna (the main part). I measured SWR along those points.
DATA:
Since Brad (WA5PSA) made a comment, please see the info below.
1. I used a MFJ-874 "Grand Master" Wattmeter. It's single needle, but the scale is big. I used the "HF" setting.
2. I checked the forward and reverse power each time.
(The batteries are getting weaker and output power is less each time)
3. I calibrated the SWR scale each time. (see above)
4. Take your time.
5. SWR= [1 + sqrt(r/f)] / [1 - sqrt(r/f)] if you want to double check the meter.
sqrt = square root
r = reverse power from antenna
f = forward power from transmitter
52.525 MHz
# in. F pwr. R SWR
1 5.75 4.25 0.1 1.5
2 5.50 5.00 0.2 1.7
4 5.00 4.75 0.3 1.7
5 4.50 5.00 0.3 1.65
5 4.25 4.00 0.2 1.85
6 4.00 4.50 0.3 1.85
RESULTS:
The antenna works at 1.5 SWR for 50.100 MHz when set up on the dot at the 4" point on the whip.
The antenna works at 1.5 SWR for 52.525 MHz when set up on the dot at the 5.75" point on the whip.
CONCLUSION:
Tuning can be tricky,
a) so keep good notes,
b) just move it little steps
c) I have to remember to cut off the excess that sticks into the coil per the instructions.
d) You have to have it on the truck where you want it when you tune it.
73
Scott
KD5NJR
Saturday, January 3, 2009
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2 comments:
I've got some hamsticks I bought at a hamfest, but I've haven't used them yet. Thus, I'm a little bit hamstick-stupid. :)
How will you handle the change in freq between SSB and FM simplex? Do you stop the truck, get out, make the adjustment, etc.? Or do you just pick one frequency and leave it there permanently, and let the other frequency suffer high SWR?
I have been wanting to set up two hamsticks, end to end, into a rotatable dipole arrangement. This would be very usable, even for HF bands, for an apartment dweller who has access to a balcony, or an attic antenna. Lots of possibilities.
What I'm going to do is insert the 20" whip 5.75" into the black coil part of the antenna and lock it down. It'll be 1.5 @52.525 and 1.4 @ 50.100 Mhz. So, it'll work.
The 4" mark yielded 1.85 @ 52.525 and 1.5 @ 50.1. So I'll use the 5.75" mark.
You're right I can't change this in flight. I don't see myself changing bands from SSB to FM on the same drive. Can only do so many things at once. One net per trip. I should be safe to QSY within a few hundred kcs, though.
I think it'll always be on FM probably. It'll be better on FM being vertically polarized. Not too sure about the activity on 6m.
Special note: The top of my quad-cab Dakota is a good ground plane.
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