Topband: Vertical short dipole ?

[Terry Conboy]

At 04:05 AM 2008-12-07, Fred Wagner KQ6Q wrote:
>My antenna (don't laugh, I live in a mobile home) is a Valor 
>PHF-160B magmounted on a metal roof, with a couple of radials - 50' 
>or so, fed through a DX Engineering MM-1.
>Since proper sized radials are not.possible, I'm wondering if would 
>radiate more effectively if I used a Hamstick Dipole bracket, and 
>set up as a vertical dipole with two Valor PHF-160B's on a pole so 
>the entire assembly was above the roofline ?
>I use a Hamstick dipole on 40, horizontally, with excellent results 
>- fed off the 25ohm tap on the MM-1 that feeds the 160m whip on its 
>12.5 ohm port.
>
>Would the Vertical Dipole with two Valor whips be worth a try ?

Fred,
I was skeptical that there would be any benefit to the vertical 
dipole configuration with two 160m mobile whips, but after modeling 
this, it appears that there is a 3 dB improvement.

I modeled the roof of your 24' x 56' mobile home roof with a 4' x 4' 
grid elevated by 10 feet and put the mobile whip in the center.  I 
assumed that the coil in the Valor whip had a Q of 400. This 
configuration produces a gain of about -18.5 dBi (about 3 S-units 
below a full size 1/4 wl antenna with a fair ground system).

I suspect the coil Q is closer to 200, since this produces a 
bandwidth near the 12 kHz specified by the manufacturer.  However, 
this would also give a feedpoint impedance closer to 25 ohms, which 
appears to be higher than what you are seeing.  The gain would be 
lower, of course.  Even though there is some disagreement here, it 
doesn't invalidate the relative results of the model.

Adding a second identical whip and raising it so the bottom tip is 2 
feet above the roof produces a gain of -15.3 dBi.  The bandwidth is 
unchanged (not unexpected, since the loading coils primarily 
determine this).  The feed impedance doubles, so you would have to 
deal with matching this (a hairpin match would be a suitable and 
inexpensive approach).  You should run the feedline away from the 
dipole perpendicular for a bit to reduce coupling.  It would also be 
a really good idea to use a choke (current) balun to decouple the 
feedline shield so it doesn't act like a radial of sorts (which it 
probably is doing now).

However, from the models, it appears that an even better (and 
cheaper) approach would be to just raise the base of the antenna and 
leave it as a monopole.  Raising the whip by 8 feet using a metal 
mast increases the gain to -11.2 dBi.  Again, the bandwidth is 
essentially the same, since the loading coil still rules.  The base 
feed Z goes up about 25%.  There is nothing stopping you from using a 
longer mast section and getting even higher efficiency and higher 
feed Z (other than mechanical limits or deed restrictions, etc.)  It 
appears that the feedpoint can either be at the junction of the whip 
with the top of the mast or at the bottom of the mast (with the whip 
shorted to the top of the mast and a suitable insulator at the bottom 
of the mast).  The gain and feed Z are similar in either case.  Your 
radials would connect at the bottom of the mast.  If the elevated 
feed is used, the feedline should be taped or zip-tied to the mast 
and run to the bottom of the mast.

I suspect your mobile home and the two 50 foot radials are a fair 
elevated ground system.  I don't think the fact that the roof and 
radials are less than 1/4 wl is a significant issue since you 
apparently can tune out any reactance by tweaking the mobile 
whip.  One would hope that your mobile home is grounded to a ground 
rod for electrical safety.  This may not be the ultimate RF ground, 
but it will serve to increase the extent of your ground system.

Let me know if you have any questions.

73, Terry N6RY