Mobile 20m 1/4 wave

[KI4YTV]

how about 2, 10-11 meter whips mounted in a v configuration ?

[VE7NOT]

In the mid 90's when everyone was into AM CB I saw a camper with a 'bigstick shakespeare' (17') on the back bumper..... (nowadays everyone runs 102" whips and uses ssb)

Since then I have looked up websites with 2200m mobile antennas... 20m is nothing...

I once causally broke into a three way qso between a ham in alaska, a ham in arkansas, a ham in houston tx and was told by them that my mobile station (a 20m hamstick with a 706MKIIG) was 5/9.

The Houston ham said I was s-9 and th the others had me at s-4 to s-5 or so....


Later that day I signed onto QRZ and got a PM from a ham on the east coast of the US (PA or NJ I think..) who said heard me....

So a 7' hamstick on 20m and 100watts from a 'simple radio' (by today's standards) was enough....

A 17 footer will be a good 1/4 wave... but put simply unless you live in area free of trees and are ready to beat up teens who make fun of your car... try a simple antenna first....

[N0EQ]

...I have looked up websites with 2200m mobile antennas...

That's ONE long antenna!
Hefty vehicle mount, to be sure!


Craig 'Lumpy' Lemke
--
Camo Battery Powered Desert Ops
www.n0eq.com

[W7XF]

As far as antenna height....

If you are WEST of the Mississippi River, the statutory height limit for any vehicle is 14'0"
If you are EAST of the Mississippi River, it is 13'6".

Take this into mind when building mobile antennas.

However, MOST newer overpasses on the Interstate system are at 16' vertical clearance minimum.

(I am a commerical driver...this is gospel)

[AD7N]

However, MOST newer overpasses on the Interstate system are at 16' vertical clearance minimum.

13.5ft is more than enough to do a bent-over antenna for NVIS. However, a full vertical would be nice, but alas those low-hanging powerlines will always give you a hard time.

[KC1HD]

Whatever design you choose, do yourself a favor and visit Alan's website on mobiling at www.k0bg.com

[W7LPN]

I've done this for a few years now. If you use a cap hat 3/4 way up it will improve radiation at the horizon. I ran several configurations and on my Dodge P/U this type worked the best. I've used auto tuners/autocouplers/impedence matching box, all with good results and the autocoupler & cap hat slightly above the rest.

[M3KXZ]

I've done this for a few years now. If you use a cap hat 3/4 way up it will improve radiation at the horizon. I ran several configurations and on my Dodge P/U this type worked the best. I've used auto tuners/autocouplers/impedence matching box, all with good results and the autocoupler & cap hat slightly above the rest.

Problem with the cap hat 3/4 of the way up is that it cuts off the top 1/4 of the antenna which would otherwise also contribute to raising the current max higher up the antenna. Unless the cap hat is massive and over compensates for cutting off the top few feet of antenna then it seems sort of awkward. Placing the cap hat right at the top would be better, or placing the cap hat at the 3/4 mark and being able to benefit from chopping the top quarter off, to be sure...

Pete

[M3KXZ]

UTI:

MUF is defined as "maximum usable frequency" and really does not apply as to how NVIS works on 160, 80/75, and 40 meters.

Now one thing that does help with the 160 meter band is the fact that there IS groundwave propagation on that band whereas on any of the higher frequency amateur radio bands groundwave does not exist. That definitely helps with the "close in" work.


HQY:

It is basically that for NVIS the antenna has to be near the Earth's surface in terms of wavelength to achieve the very high angle of radiation required. On the 160 meter and 80/75 meter band this is very easy to obtain and fairly easy to obtain on 40 meters. But, by the time you get to the 20 meter band the antenna is going to have to be almost on the Earth's surface. On 20 meters a quarter-wave is less than 18 feet and then you have to consider that in most soil types actual r.f. ground is between 6 feet and 8 feet below the surface. That means a 20 meter antenna 10 feet above the Earth's surface is a quarter-wave above r.f. ground and that is a reasonable height to start getting a decent angle of radiation. To get the very high angle of radiation from a simple antenna that means the antenna is going to be only a foot, or two, above the Earth's surface.

Now if you were to point a yagi straight up then you will get the high angle of radiation necessary for NVIS. But, how many amateur radio operators point their yagis "straight up"?

Glen, K9STH

I know this is an old thread....but I think there is some misinformation here. Glen, what makes you think MUF doesn't apply to NVIS communications on 160, 80 and 40? The MUF is VITAL to how NVIS communications work on these bands - MUF is the critical frequency above which radio waves are not reflected by the F2 layer when the angle of incidence is vertical. If the MUF is lower than the frequency you are using, then NVIS communications will not work, as the RF will go straight through the F2 layer and into space rather than being reflected back down to earth. And as for 20m, well unless you live somewhere where there is often an MUF greater than 14MHz, then that's a complete no-no. I don't know quite why anyone would want a high angle of radiation at 14 MHz.

Now when looking at longer paths, where the angle of incidence is nowhere near vertical, then the figure to look at if the MUF(D), for whatever length of hop you're looking at - i.e. the map might show an MUF of only 5 MHz at a point halfway along the path, but you could well have a MUF(D) at that point for a 1500km hop (for example) - known as MUF(1500) - of 14MHz.

[M3KXZ]

I am still curious what makes 160m/80m/40m bands better for NVIS than 20m. Is it band propagation? takeoff angle...?

As UTI said - it's the MUF. Have a look at the ionospheric maps here http://www.ips.gov.au/HF_Systems/6/5 . Look at where you're location is, and see what the MUF (maximum usable frequency) is. You need to be operating below the MUF for your very high elevation radio signals to be reflected back down from the F2 layer. The higher the frequency you can use, while still being below the MUF, the better as the D layer absorption is lower.

The good thing about running an antenna for 160, 80 or 40 fairly near to the ground for local comms is that the antenna ends up pretty deaf to signals from further away, and you get a much better signal to noise ratio for the closer in signals. It works a treat and is the reason why local (out to a few hundred miles) ragchews are so easy on the lower bands.

Pete