The AM Forum

For reasons I won't go into I'm going to have to have a sloper antenna. This would be a 20 meter center fed dipole with a balun. It would be oriented north to south with the high end on the south. approximately 10 to 20 feet. All fed with LMR400 coax.  No radials. I should note that I am at 4500 feet in a valley with tall mountains -- 5000 to 7000 feet surrounding me in all directions except due north.

From my reading I think this would give me dominant lobes east and west. Any thoughts on this?

K,

Beware the Blob.

A 20m dipole at 20' will have most of the energy going straight up into the clouds....  The proposed sloper may have some directivity, but probably not much. Do the angle 'o the sloper approach 90 deg?? I would guess the closer to 90, the better.

Maybe try for a 20m vertical to give yourself some low angle radiation.

Free advise worth every penny.

KLC

YUP!  That's a description of a NVIS antenna.  You need to be 5/8 to 3/4 of a wavelength above ground in order to realize classic patterns be it a classic dipole or a sloper

Low angle of radiation?  Try a 20 m vertical with loads of radials

Here's a nice web site to check out:   http://www.qsl.net/aa3rl/ant2.html (http://www.qsl.net/aa3rl/ant2.html)

Al

That's interesting since I have been using my dipoles -- at 14 to 20 feet -- for both 40 and 20 meters as an NVIS antenna. I have good solid contacts within 400 miles, but, and I have no explanation, I also have a reasonable amount of distance contacts. Last weekend I contacted France on 20 meter SSB with 5/9 signal reports.  Also Hawaii, Massachusetts, Alaska, California, Oklahoma, Minnesota,and South Carolina -- with  good reports. British Columbia also.  I routinely am hearing places like Costa Rica, Mexico, Australia, and Ukraine but have little success breaking the pileups.

This with 80 watts into a center fed dipole at no more than 14 to 18 feet. I did put two radials -- one under each leg. 

I'm quite puzzled by these results since the antenna is neither high enough to function as a proper dipole (33 feet or more) yet I am getting non-NVIS results with an NVIS configured antenna. This is from Idaho at a height of 4400 feet surrounded by mountains of 5000 to 6000 feet. I don't know exactly what is happening with this only that it works.

If you look at patterns for your antenna(s) you'll probably find that there is some radiation at lower angles. And this will give you your  "Dog Xray" contacts.
My 80m dipole is only ~45 feet above ground, but I can make contacts in Utah, 2000 miles away. Like Platformate, your mileage may very.

A low 20m antenna, and/or sloped will have mostly an omnidirectional azimuth pattern, and will favor high angles.

**It will NOT be a "NVIS" antenna since the ionosphere does not support NVIS much above 10 Mhz on good days and in these poor times, often doesn't support it on 80m!!!   High angle rays just shoot on through the ionosphere and continue into space.  They are not reflected or refracted back to earth like low band frequencies.

I ran an EZNEC model for 20m dipole sloping from 1 foot to 20 ft.  Here are diagrams.  Wire model; Elevation pattern; Azimuth pattern.

Good reading:   http://www.sws.bom.gov.au/HF_Systems/6/5

I respectfully disagree. 

Back a few years ago when messing with antenna designs I found the MMANA-GAL application very useful and easy to use.
http://hamsoft.ca/pages/mmana-gal.php

73s  Nigel

I should probably clarify my previous "respectfully disagree.  Regarding NVIS propagation on 20 meters.  Yes, NVIS is not normally supported very well if at all.

The antenna should be around 3/4 wavelength to get some useful lobe angles low enough to utilize long distance propagation..  I'm suggesting the antenna height as proposed will produce disappoint results. 

May I suggest an antenna height of 50' for good low angle lobes.

Al

Thanks. I greatly appreciate this. I moved the antenna yesterday so I will try and figure out this weekend how well this is working in its new location.

I agree higher is better. Unfortunately the 100 foot tree on my property is in the front yard. No way I can put an antenna on it given zoning issues.

For those with the opportunity, the best simple, wire, 4dB gain 20M "dipole" antenna one can put up is a pair of stacked dipoles. ALL high angle gets suppressed and gets directed into a figure eight pair of low angle lobes at about 22 degrees takeoff angle or so. Inverted Vs will work too.

Hang one dipole at 66' and the other at 33' directly below it. Face them broadside in your best directions, just like any dipole.  Use the same length coax feedline for each dipole and tie them together. The coax lengths are not important as long as they are the same length. This ~30 ohm input impedance can be matched to 50 ohms or you can just live with a 1.8: 1  swr.

This is a killer antenna for 20M and will compare with the common lossy trap Yagis out there. (except it is bi-directional)  Add a pair of 35' long wire reflectors spaced 16' behind it and be uni-directional with ~6.5db gain and 20dB F-B. The equivalent of a pair of 2el stacked full-size 20M Yagis. BIG SIGNAL.

Al/VTP, if you get a chance, try modeling that and posting it. I'll bet there are some out there who will be motivated by the pattern enough to give it a try.  It can be tweaked more by using 40' stack spacing (5/8 wave) and optimizing the reflector lengths.

T

Stacking is the bomb!

If you cannot do that, consider a rectangular loop (approx 1 wavelength circumference). It will not have the gain of a pair of stacked dipoles but it will beat a single dipole by a few dB. It's a poor man's stacked dipole system. Info on one I built for 10 meters below. I also built one for 20 meters.

http://amfone.net/Amforum/index.php?topic=30353.0


You can increase the gain to about 3 dB over a dipole by increasing the circumference to 1.3-1.4 wavelength. The downside is that the feed point is no longer 50 Ohms (more like 70 + j600). A simple matching network would take car of this or feed it with ladder-line and use it on multiple bands.


Some good info on stacking at the link.

http://www.qsl.net/dk7zb/start1.htm

This looks really interesting. I wonder -- This design uses a mast which I don't think is involved with the antenna -- it's just a support. If you put two eyebolts in the top you could just hang this from an upper support e.g. horizontal tree limb. You wouldn't get much height, usually, but it does avoid having to install a ground mounted mast.  Say for field day use -- tie ropes to the eyebolts, toss the ropes over the limb and secure on the ground with rocks or concrete blocks.

 I have a reasonable amount of PVC piping available which I intend to use for this when it gets somewhat warmer.

I didn't use a mast - just a piece of PVC pipe at the top and bottom. Top PVC hooked to a rope and pulley on a steel cable. Steel cable went of the top of the tree(s) branch(es), as appropriate.