Interesting discussion on inverted V's. I have probably a typical inverted V
setup for 80M strung off of a tower. I have the center of the V strung off my
tower at 90 feet and the ends slope down to 35 feet to two poles on each side
of my property. Here's an idea I have been thinking about. Hanging a
vertical dipole from the tower with loading coils in each leg to compensate for
the shorter length. Better low angle radiation ? I know I would have reduced
bandwidth but that would be ok, I hang out in the very low end of 80 anyway
chasing CW DX. What do you think, improvement over the V ?
Bob
K6UJ
> On Jun 15, 2020, at 7:10 AM, jimlux <jimlux@earthlink.net> wrote:
>
> On 6/14/20 8:23 PM, Jim Brown wrote:
>> On 6/14/2020 7:03 PM, wesattaway wrote:
>>> However, as overall height is raised then best performance occurs when the
>>> wires are level. I think Jim Briwn may have some data on this.
>> Hi Wes,
>> My study was on the effect of height on horizontal and vertical antennas,
>> and I developed a figure of merit in dB for height of horizontal antennas.
>> The executive summary is that for 30M and below, higher is better. :)
> <snip>
>
>
>> 3) Soil quality STRONGLY affects vertically polarized antennas -- the better
>> the soil conductivity, the better they work.
>> 4) HF verticals work better on the roof than on the ground.
>>
>
> <snip>
>
> There's two separate factors at work in #3
> a) a "near field" effect - for a monopole vertical, the ground (or radial
> field) is half the antenna. Hence the "120 radials" for FCC proof of
> performance exemption. Not so much effect for a vertical dipole.
>
> b) a "far field" effect - H-pol is reflected well almost at any incidence
> angle and with any soil properties. Not so with V-pol which is strongly
> affected by soil properties and incidence angle.
>
>
> The difference in these two effects (in broad strokes) is that (a) is a big
> deal close in (dimensions comparable to antenna height) and (b) is about the
> soil properties farther away.
>
> Consider a 50 foot tall monopole. You can think about the ray from the
> antenna hitting a spot at some distance and then reflecting. And each point
> on the antenna hits a different spot.
>
> For a low elevation angle, say, 10 degrees, the spot for the top of the
> antenna is 50/tan(elev) = 283 feet away. And it gets way farther out very
> rapidly. For 3 degree elevation, the "reflection spot" is 1000 ft away. Of
> course, for a spot on the antenna that is 25 ft high, the "spot" is half as
> far away.
>
> So for really low angle radiation (like 3 degrees), everything within 20
> times the height of the antenna contributes.
>
> Hence the popularity of verticals at the beach, or in the middle of the
> proverbial salt marsh.
>
>
> As Jim points out in #4, raising the antenna is good (reduces losses from
> near field (a)) but does extend the far field issue. For a 50 foot elevated
> dipole at 100 ft the radiation at 3 degrees is reflecting from spots at
> 1500-2500 ft away.
>
>
>
>
>
>
>
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|