[Antennas] need help with HF mobile mag mount

[George, W5YR]

Dave, it seems to me that the cross-polarization loss issue is of
importance only when ground-wave and direct-wave propagation are
considered. After the first ionospheric contact, polarization tends to
become randomized for the remainder of the path, especially as subsequent
Earth and ionospheric contacts and transitions are involved.

I use several horizontally polarized antennas together with my vertical and
outside of local groundwave contacts I have never been able to see the
results of a polarization change among antennas. A signal coming from a
horizontal antenna beyond groundwave will usually be as strong on my
vertical as on my horizontally polarized antennas, and vice versa. I have
also found this to be true even on 40 meters at distances as short as 25
miles. On occasion, I have found that the vertical actually provides a
stronger received signal than a 40 meter horizontal antenna, probably a
signal-angle-of-arrival situation.

I also wonder how much of that 5.3 dB typical loss you mention for the
mobile antenna on 20 meters is due to ground and associated loss as well as
to a shortened radiator. As you know, provided that the power can be
delivered to an antenna and that its internal losses, including ground
loss, are taken into account, an antenna can be quite short indeed with
little loss in field strength. The theoretical figure is about -0.45 dB for
a dipole of infinitesimal length compared to a full-size dipole.

Of course, the practical issue is that the driving-point impedance of a
short antenna comprises a very low real part and a very large capacitive
reactance. Thus, the power lost in the impedance matching network and the
antenna structure itself offsets any lack of loss of radiation efficiency
due to the shortened radiator. But, as Jerry Sevick demonstrated in the
70's, quite short vertical radiators on 40 and 20 meters produced field
strengths very comparable to full size antennas. Of course, he used an
extensive radial field in these tests to virtually eliminate ground loss.

I appreciate that your remarks are biased by your extensive antenna range
experience where polarization is of primary importance in direct-path and
ground-wave propagation.

73/72, George    
Amateur Radio W5YR -  the Yellow Rose of Texas
In the 57th year and it just keeps getting better!
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe
K2 #489      Icom IC-765 #2349     Icom IC-756 PRO  #2121


Dave Shrader wrote:

> 1) You loose about 3 'S'[~ -18 dB] units due to cross polarization when
> compared to a horizontal dipole. You loose an additional 'S' unit [~
> -5.3 dB on 20 meters] compared to a dipole because of the shortness of
> the antenna.
> 
> So, if you are comparing the same signal as heard at your home QTH on a
> dipole and the same signal in the mobile a change of -3 to -4 'S' units
> is typical and normal. Or, if a friend is listening to your signal from
> the home QTH and comparing it to the mobile you get the same result. A
> mobile antenna on 20 meters is not as efficient as a full size dipole!
> 
> In conclusion, all mobile antennas for HF suffer from cross polarization
> losses [~ - 18dB] and shortened antenna losses [smaller effective
> radiating cross section][~ -5 dB on 20 meters].