Topband: Mixed elevated and ground located radials?
grants2 at pacbell.net
Mon Jan 29 12:02:13 EST 2018
Radials need to be elevated and insulated from ground/earth or on the
ground and grounded. I tried a modeling exercise to see if a ground
screen under elevated radials would improve efficiency but it wasn't
very helpful. Even over salt water, it is better to insulate elevated
radials or place them in the salt water at the surface, not both.
While salt water is about 1000 times more conductive than "average
ground", copper is about a ten million times more conductive than salt
water and then RF penetration is shallow.
I use 8 ten foot elevated 125' radials for 160m, pretty much symmetric.
Because of proximity of a couple of the elevated radials to steel
structures (tower and steel building) the measured currents are
significantly unequal, higher towards the steel structures. The
conventional wisdom strongly recommends equal radial currents but the
analysis of why seems thin. Calculations show negligible loss from I^2
differences. So I modeled very unbalanced radial patterns and found
only a db or two of pattern asymmetry. Also, how one would get currents
more equal in my situation is a mystery to me, as well as how to model
radial fields with known current imbalance without introducing loss.
Still seeking advice on these matters, although the unbalanced geometry
modeling makes it less of a concern. see Rudy Severns work
Your peak gain angle is more affected by the wider area far field ground
conductivity, whereas antenna efficiency/gain is more dependent on the
radial system and ground conductivity out to 1/2 wavelength. For
verticals within 3/4 wavelength or over salt water, the peak gain angle
gets within a few degrees to the horizon. Even with 2 tuned elevated
radials. see my article "Verticals on the Beach" QST June 2016. If
your marsh is salt water, you have high gain low angle propagation in
that direction, although at a sacrifice of gain in the opposite
direction. If it is a fresh water march, then it might be better or
worse than "average ground" depending on the chemistry/conductivity.
On 1/28/2018 12:08 PM, Gary Smith wrote:
> Now that I have both a HI-Z circle 8 & a
> triangular array, along with the K3s, I
> can hear most stations coming in around
> here. About the only thing that will help
> me Rx wise at this time would be a better
> QTH more suited for Ham Radio than this
> RFI laden environment.
> This last 160 contest made me realize my
> best option till I move is to try and get
> a lower angle on the Tx. I have maybe 45
> 130' radials attached to the radial base
> and am using WD-1A wire for my antennas.
> The antennas are being fed by lengths of
> coax from the remote switch to each
> specific antenna. At the end of each coax
> I have about 12" of braid & center
> conductor separated with the braid to the
> plate and the center to the antenna.
> Crude, but it works and seems to be the
> best I can do here.
> It's a challenge to get any more radials
> down at this time and because of the marsh
> behind me and the radial plate at the edge
> of the marsh, I can only put any new
> radials in a semi-circle. but I've been
> wondering if I could add elevated radials
> to what I have? the feed point is now only
> a foot above ground so any elevated radial
> would be above the feed point. What might
> I expect if I were to add say 3-4 elevated
> radials about 8' or so high? I'd have to
> run them that high to keep the deer from
> taking them down. If I made them brush
> high, they'd get snagged right away.
> Would it be worth my effort? My mobility
> is reduced as of late.
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