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writes: > OK, we are in the final stages of tuning the 2 el 40M portion of my C-4XSL > (by the way, the 10-15-20M portion really kicks). The MFJ antenna analyzer > indicates that we are loading at around 6.800 MHz, so we are way low in the > band. The FT1000's SWR meter also indicates a high reading, but since the > resonant frequency is out of band, we cannot use the the rig to tell us > exactly where we're resonant, except by tuning it to the MFJ's output). What this is showing you is that's where the driven element resonates. Each element of the Force 12 linear loaded antennas need to be tuned individually - that's why the manual gives you the resonate frequencies for all the elements. At this point you don't know what the resonance of the reflector is. It needs adjusting too. The measurements in the manual in my experience are just starting places for tuning - rarely are they on-the-money. While the most important SWR measurement is how the antenna looks at the rig, the MFJ analyzer can be used for both elements. I would suggest using the analyzer at the feedpoint from on the tower to adjust the DE. That at least will get you half-way there. If the antenna is installed on the tower, tuning the reflector is somewhat problematic and may produce poor antenna performance if it's not right. There are several schemes for adjusting the reflector depending on the height of the antenna but that's a topic for another email. > We've opened the 3 turn coil about as far as it will go (closing it lowers > the resonant frequency), Hmm, the coil shouldn't do anything to the resonance because you're not changing the length of the element; the coil is basically a hair-pin match in a coil form. It should affect the SWR only. If it's actually changing the resonance, something is wrong. > and we've also tried moving the jumpers on the > driven element (and only on the side closest to the mast) out around 2" more > toward the element tips from the 12" presets. These action have resulted in > a noticeable but nominal impact on the resonant frequency. Well then, keep moving them out until you get the resonance you want. > > We also have looked closely at the photos in the tech note for > assembling/tuning 40M on the C-4XSL, which has led us to pose the following > questions: > > 1. When the alumoweld wire is brought through the center spreader and folded > back, should the folded back portion be held by BOTH clamps--that is, by > both the clamp that initially holds the end of the alumoweld wire to itself > AND the clamp that holds the jumper? The 2 clamps are for different purposes and I'm not sure if there is any problem with putting the jumper under both but then it makes it very difficult to adjust the jumper which is what you're trying to do. > Our thought was that by clamping in two > places, this would reduce the size of the loop formed by folding back the > alumoweld wire, and thereby possibly reduce the electrical length of the > element (since we're now clearly too long to be resonant within the 40M > band). I've found that you can slide the jumper enough from the tower to make it resonate where you want. I've never had to change the size of the wire jumper. > > If this approach is required, I assume this should be done on both the > reflector and driver? > Yes - they both need to be tuned but you should be able to do it by moving the jumper. > This is the only thing we could think of in reviewing the photos that seems > different from what we've done during assembly, and the comments to the > photos seem to suggest the same thing--clamp the folded portion of the > alumoweld with both clamps. One clamp is just mechanical to hold the linear loading wires and the other is electrical to adjust the element resonance. > > 2. If the above approach is not required, does anyone have any ideas for why > we are loading so low out of band? Sure - the element needs to be shortened by adjusting the jumper. > The leads from the F12 balun are not > unusually long (around 3", an adjustment of which shouldn't make a big > difference on this band). The antenna is on a crankup that we took to full > height to make sure we are clear of interactive items like the roof. We > originally assembled the 40M elements to emphasize the CW portion of the > band, following the presets in the manual (3" away from the spreader for the > reflector, 12" away from the spreader for the driver). Should we be looking > to move the jumpers out quite a distance on both elements (even thought that > would suggest the SSB preset per the manual)? The resonance of the reflector may be affecting the DE. Tune the reflector and then the DE and everything will be fine. What's 'quite a distance'? An adjustment of 6 inches or more may be required. > > 3. When making tuning adjustments, do we need to adjust the jumpers at all 4 > points of connection to the alumoweld wire, or can we make the adjustments > on just the one side of the 40M driven element that is closest to reach > (these would be the points of connection that are on either side of the > boom, but on the same side of the 40M element)? After finding the resonance with the analyzer, slide the whole jumper one inch and see what happens. Repeat until you're satisfied. > I know, the manual says > adjust them all, but I thought I'd seen some suggestions here earlier that > one could make the adjustments on just one side of the element and be OK. > The LXC Prime Directive states to "DO what the manufacturer says". > Do we need to include the reflector in these adjustments, or just the driver > (again, the manual states the driver only, but want to make certain). > Both. Cheers, Steve K7LXC Tower Tech -------------------------------------------- Force12Talk mailing list provided as a service by Force 12 Antennas, Inc. Web sites: http://www.qth.com/force12 and http://www.force12inc.com Submissions: send to Force12Talk@qth.com To unsubscribe: send a blank e-mail to Force12Talk-leave@qth.com Force12Talk Message Archive: http://www.qth.com/force12/list/force12talk For problems with the list, contact n4zr@qth.com |
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