[TowerTalk] Thanks - and follow up Q's (long)
n6ry at arrl.net
Fri Jun 23 20:38:53 EDT 2006
At 03:15 PM 2006-06-22, Eugene Hertz wrote:
>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.
>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?
This should have a fairly omni pattern on 160, 80, 40, & 20m with
good low angle signals. On 30m, 17m, 15m, 12m, and 10m, where you
don't have a 1/4 wavelength conductor, the longest wires are NOT
decoupled and will radiate with odd patterns. In addition, you need
to realize that you are trading the convenience of one antenna for
performance above 40m, since horizontal antennas at reasonable height
normally outperform ground mounted verticals.
>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.
As long as the insulation will handle the RF voltage present at the
open end of the active conductor, it should be OK. Losses will be a
little lower with more air between the wires. Impedance bandwidth
will normally be larger with more spacing, but your tuner should be
able to handle the changes in reactance. (I've had good performance
when spacing only the ends of the wires (the classic fan) on inverted vees.)
>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?
There will always be some loss in a trap, due to finite Q's of the L
and C. With a good trap, the losses will be minimal, and confined to
80 and 160m. The bandwidth can be larger with a separate wire, but
again, your tuner should deal with that. An alternative would be to
use 300 or 450 ohm line and cut one conductor at the ~67 foot
point. Beauty is in the eye of your XYL, of course.
>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.
The presence of the 80m trap will add inductance to the 160m wire,
and probably lower the resonance below 160m (if you use the 130'
overall length). According to EZNEC, using maximum wire appears to
slightly reduce the low angle signal (less than -1 dB, depending on
ground loss) and increase the high angle lobe a bit (good for close
in stations) on 160m.
>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?
Grounding is harder the farther you are from it. The proximity of
your antenna to the house may induce a lot of RF current in AC power
wiring and metal plumbing, which CAN be a pain. (I've had
incandescent lamps in another room get brighter when I keyed a 200w
TX connected to an attic antenna.) Have you considered putting the
vertical portion of your antenna near the tree to get it farther from
the house (assuming it's not too close to a neighbor's home)? You
could run coax out to your remote tuner.
>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?
Placing the trap is equivalent to changing the length of the wire up
to the trap. Modeling should get you close, assuming the metallic
stuff in your house isn't too tightly coupled. Again, the tuner
should compensate. Trial and error isn't too hard with an SWR
bridge. Without the trap, the voltage node will be about 1/2 wave
from the open end of the wire, however you need the trap about 1/4
wavelength from the feedpoint, which probably aren't the same.
>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
>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?
I'm sure you're right about the impedance of a short whip being the
reason for the step-down transformer. It's probably helpful on 160m,
but may be counterproductive on higher bands where the impedance is
closer to 50 ohms (when ground loss is included).
As for the series 50pF cap, I'd bet you're right about it getting the
impedance into the range of the tuner. In all likelihood, adding L
or C will make it easier for your tuner to cope, but without more
info on the range of the internal tuner components, you can't say
exactly what will work best at a given frequency.
73, Terry N6RY
PS Eugene's mailer/editor inserts strange codes for apostrophes and
quotes. I edited most of them out.
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