[TowerTalk] Thanks - and follow up Q's.
ehertz at tcaf.org
Thu Jun 22 18:15:13 EDT 2006
Super thanks to everyone who responded to my questions. Not only did I learn all about the pros and cons of antenna wire, not only did I get some fill-in-the-gap knowledge about radials, but as an added bonus, I even got some really great, exciting and unexpected advice about the antenna itself!
Several folks gave some great ideas about how to improve this L. My take-away from this is that even though the tuner could probably load this antenna, my angle of elevation would be less than ideal for many bands. So I took some ideas of several people on this list and this is what I came up with, please see this diagram:
The idea is to run 3 wires out of the antenna tuner. One 17’ long, one 33’ long and the third 130’ long with an 80m trap located at 65’-67’ out. The trap would allow some aesthetic improvement (for the xyl) and keep only one wire running the length across the front yard. This antenna should not be any more difficult to erect than the plain L. Need to insert a trap and run a two more ropes to hoist the 17 and 33’ sections, but that’s it. If this would be much more beneficial than a plain L, then this would seem to be the way to go.
So, a few questions!
1. How might I expect such an antenna to perform? Will I get “more appropriate” angles of elevation on the different bands than the original, single radiator L?
2. Can the 17’ and 33’ sections run completely next to each other? I am planning to use insulated wire. Would these need to be spaced in some fashion or can they just rub up against each other and work? I was only familiar with the fan concept on dipoles, and didn’t realize this was a possibility with the L. One of the things that always scared me about the fan dipole was the diagrams always showed the radiators “fanning out”, not running right next to each other. That always seemed a headache to mount. If they can all run next to each other without separation (other than insulation) then this becomes very easy to erect.
3. The thought was to use a trap between 67’ and 130’ so I don’t have two wires/ropes running across the front yard, inevitably sagging at different points and looking ugly (that’s xyl-speak!) Question is, the 80m trap. Will that cause any significant degradation or loss? Will this loss occur only for 160 or 80? Or perhaps the other bands? Would there be a noticeable difference in performance if I used a separate wire for this instead of a trap? Or would the difference be minor?
4. You will notice that the 160m wire [C+D] at a length of 130’ in the diagram would still leave me with about 48’ of rope from the insulator to the tree. Is there any other approach I could take to make more use of this wire? One nice gent suggested that I use the entire length for 160m (namely 180’) and let the tuner do the rest. Pros? Cons? Hate to not make use of 48’ ! don’t know why, just seems a shame
5. One fellow suggested that I should expect to have problems with RFI and ground loops (ouch!) because my shack is on the 2nd floor. I was worried enough as it is if I were actually going to be able to create and put up an antenna like this in the first place. Now I am worried that after all that trouble, there will still be more hoops to go through to avoid some of this other nasty stuff. Advice?
6. Tuning the wires. How critical is the placement of the trap? Can the main tuner at the feedpoint compensate for any misplacement of the trap? Other than trial and error, is there any more precise way to place the trap? For example (please don’t yell at me, this is strictly an academic thought), could I run the transmitter at very low power (1 watt, for example) and use some kind of rod with some kind of meter, move the rod along the length of the wire and locate some kind of voltage node (either high or low or something) and that would tell me where to place the trap?
One last comment, the tuner I have was designed for 15' to 35’ vertical whips. I believe that the 50 ohm transmitter is immediately converted to 22.2 ohms using an internal transformer. Then the tuner elements take effect (again, if interested, see <http://www.torontosurplus.com/com/harris/harris_rf601a.htm> of particular interest, see figure 4-2) I think this has something to do with the nominal feed point impedance of a 35’ whip on board a ship, but I am not 100% certain. I will point out that Harris made a “long wire kit” for this tuner that was essentially a 50pf (very high voltage) capacitor to be added in series with the wire. I am too green to fully understand the impact of this capacitor, but I am guessing it sorta stacked the deck a bit to more easily accommodate the nominal feed point impedance of a horizontal wire vs the whip. Reason for all this discussion: Several folks have pointed out (and one nice gentlemen has experience using this tuner) that its range is limited to 2mhz (if that low). Question is, can I add a fixed capacitor or inductor to help the tuner match the impedance encountered at a lower frequency of 160m?
Anyway, that’s MORE than my fair share of questions for the week!
Thanks again to everyone. This list blows the other antenna related lists away when it comes to so many people offering so much quality advice.
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