Antennas For the Remote Base

The equipment is behaving and it’s time to turn our eye toward antenna improvement and more gain for DX.  Our original intention was to start with the PAR/LNR EF-20 on a 39′ fiberglass Spiderbeam pole to get on the air fast.  This is the antenna that has proven itself in multiple locations over the years and it has worked well here so far. Now all we want is more.

2016-03-02 14.32.27

How to improve?  First of all the EF-20 works only for 20 meters. Any antenna that gives us more bands will be a big improvement.   Next step is to find more gain at low angles for a better DX antenna. Horizontal dipoles at the proper height are always better DX antennas despite verticals’ claim to low angle of radiation. Verticals only win if they are over salt water. For a full size quarter wave vertical over average ground a gain of 0 dBi is typical and much less for a shortened vertical.  Horizontal antennas have the advantage of gain from ground reflections which verticals don’t have.  That gives dipoles about 8 dBi more gain above average soil.  Our goal is to aim that 8 dBi at the angle we want by putting the dipole at the correct height.

What is the correct dipole height to achieve DX?  First we have to determine the take off angle and that is a function of geometry — the distance to the DX station and the height of the ionosphere at that given moment.  The angle changes minute by minute and station by station making it impossible to select a perfect angle.  We can look at tables and computer programs to come up with a range.  Very distant stations require angles as low as 3 degrees and stations fairly nearby can be worked with 40 degrees.  Accumulated wisdom seems to say the best compromise to work the most stations is 15 degrees and we will call that our optimum take off angle.  This is also the angle we will use for our calculations.

What is the necessary height to attain a 15 degree take off angle for a 20 meter horizontal dipole over average ground?  We have modeled multiple dipoles using EZNEC 5.0 to find the right one. It’s strongest lobe at 15 degrees is when it’s height is 64 feet. EZNEC assumes flat ground which we don’t have. We also haven’t measured our soil conditions although we have referred to the M3 Map published by the FCC ( and reprinted in the ARRL Antenna Book ). Because of these two approximations our models will only be representative and not perfectly accurate.  We could make a soil conductivity test some day and improve our accuracy. A method without driving spikes is presented in the 23rd edition of the ARRL Antenna Book.

Figure 1 – “Ideal antenna”

Screenshot 2016-03-27 14.48.49

Here we show a gain of 7.22 dBi at 15 degrees, which is also the angle with the maximum gain.  Thus we know we are at the proper height for maximum gain at 15 degrees. In addition there is a healthy lobe at 50 degrees for those close-in stations. Notice the  nasty null at 35 degrees. We’ll call this the ideal antenna except for geographic areas where the 35 degree angle is needed. It’s really not an ideal antenna at all.  It just has a maximum at the 15 degree angle we want.

Now, let’s see…could I put up a 64 foot dipole….hmmm.  Not totally unheard of using a pair of Spiderbeam 18 meter fiberglass poles.  For now we will keep the “ideal antenna” in mind only as something to compare to.

For purposes of comparison let’s model the EF-20 vertical we are currently using.

Figure 2 – Existing vertical antenna

Screenshot 2016-03-27 15.49.37

Yes, it has a low angle of radiation peaking at only 17.6 degrees like verticals are suppose to.  Unfortunately the gain at that peak is only .71 dBi.  At 15 degrees, gain is only .5 dBi which is 6.72 dB worse than the dipole.  Pitiful.  It’s a wonder we’re working any DX at all.  We’re giving up over 6 dB from our ideal antenna.  We need four times the power just to break even.  Instead of 10 watts we need to run 40.  Ouch.  Again, on the other hand,  not unheard of.  That’s only down one S-unit.

Tom Rauch, W8JI, makes another point in an article titled “The Myth Of Take Off Angles”.  He suggests taking a more holistic approach and don’t ignore nulls and missing angles of radiation.  Look at the whole pattern. An antenna with deep nulls and missing angles is going to miss DX from a lot of geographies. The dipole is missing everything that requires a 35 degree take off angle. The vertical has no nulls but it is missing the entire higher angles from 30 degrees on up.  That will make it hard to work stations close in.


 

Are higher angles essential?  Power must be increased by it’s square to double the distance. That is, we need to quadruple the power to double how far we reach.  Another way to look at it is we only need a quarter of the power to work close in stations which is 6dB less power.  Or our antenna can have a 6 dB reduction in gain at the high angles when using the same power.  Perhaps a pattern like Figure 2 is not such a problem after all. 

 


What heights are within our means?  We might possibly use surplus fiberglass camo poles to reach up to a half wave on 20 meters, 32 feet. We would need two supports of 8 poles each. Here is what our gain would look like at 15 degrees.

Figure 3 – 20 Meter Dipole At 32 Feet

Screenshot 2016-03-27 15.24.52

Wow, 4.03 dBi vs. 7.22 dBi at 64 feet.  That is quite a hit.  More than 3 dB is given up. That’s half our power.  We could live with this by just turning up the power.  If we were going to run 10 watts we could crank it up to 20 watts and break even.  We might have to settle for this. Notice the maximum gain is 6.74 dBi at 31 degrees.  That angle is going to maximize signals to the U.S. East Coast where a concentration of hams live.  Trying to reach Europe competing with those strong East Coast signals could be a problem.  This is a real negative for this particular height.

Next let’s look at a little more height. If 32 feet uses 8 camo poles how about adding two more poles for each support to reach 40 feet?

Figure 4 – 20 Meter Dipole At 40 Feet

Screenshot 2016-03-27 15.35.24

Hmmm: 7.22 dBi – 6.15 dBi is down only about 1 dB. Still a loss but a 6 dBi gain at 15 degrees is nothing to sneeze at. That’s quadrupling our gain over the existing vertical plus there are no severe nulls. We have a peak at 24 degrees and that might be low enough to put the first hop beyond the East Coast, minimizing the strong competing signals.  This might be a compromise we can live with. Two supports of 10 camo poles each could be quite an undertaking.  Still nothing compared to the work involved of putting up a tower though.  A few years back we put up a loop at 28 feet using 7 camo poles for each end.  It looked like this.

image1[1]

These 7 section poles stayed up through winter and summer weather for a year and a half with no issues before I took them down.  But what if the supports were 10 sections high instead of 7?  Would it survive?

Would it be possible to eliminate one of the two support poles and put up an inverted vee on just one support?  Here’s a model.

Figure 5 – 20 Meter Inverted Vee At 40 Feet

Screenshot 2016-03-27 10.35.35

We’re back to that slightly disappointing 4 dBi range and we have that dreaded peak toward the East Coast again.  Compared to our ideal horizontal dipole at 64 feet with a gain of 7.22 dBi at 15 degrees we are giving up 3 db, half our power. We can overcome that by cranking the output but the real problem is those East Coast signals overpowering the European stations.  This is similar to the model of a dipole at 32 feet using two supports.  At that time we said we might have to live with doubling our output power to break even.

Conclusion

If you got this far, congratulations!  It’s a lot of boring graphs and reading.

Any of these horizontal antennas more than doubles the gain of the existing vertical but would they double the performance?  The existing vertical has one thing going for it. It’s low angle minimizes strong signals from the East Coast that can wipe out weak signals from Europe.   Table 1 summarizes the work.

Table 1

Antenna               Description                                           Gain @15 deg  Peak gain


Ideal antenna     horizontal dipole up 64′                    7.22 dBi           15*

EF-20                    end fed half wave dipole vertical    0.5 dBi             17.6

Dipole                   horizontal dipole up 32′                     4.03 dBi           31

Dipole                   horizontal dipole up 40′                    6.15 dBi            24

Inverted Vee       inverted vee up   40′                           4.78 dBi            27

*Peak gain is at 15 degrees but there is large lobe at 50 degrees that allows strong U.S. signals to over power the DX.

Multi Band

Multi-band operation hasn’t even been mentioned.  For other bands the plan is to use this antenna for all the high bands 20 thru 10 meters along with an SGC SG-230 auto tuner at the base.  The transmission line from the tuner up to the feed point would be ladder line.  For the lower bands we would use the transmission line as the antenna and the inverted vee would become a top hat.  For radials we would lay out welded wire fencing on the ground.  A relay would switch between the low bands and the high bands controllable over the Internet, of course.  The SG-230 would be used for both high bands and low bands.

A lot to digest and we’ll need some time to think about the best compromise. I have no decision for now.  Thank you for enduring this long post.

Post Note

At a hamfest yesterday I picked up a multiband vertical.  It’s a Comet CHA250BX.  It is 23.5 feet long with a very unique matching network at the bottom.  I think it is attempting to be an end fed half wave on each band. The coax provides a counterpoise. I plan to use a non metallic mast so it doesn’t interfere with the coax radiation.  Based on the model below I do not expect good things.  It’s advantage is it’s multi band  and it will be easy to put up considering it doesn’t need radials.  It might be a good interim antenna until I decide what to go with.

Figure 6 – Comet CHA250B Vertical Antenna – 20 meters

Screenshot 2016-03-27 19.58.00

 

March 28, 2016 Followup –  After sleeping on it and looking over yesterday’s work again I’m more confused than ever.  The existing EF-20 has the advantage of minimizing U.S. interference but the gain is very low.  Boosting power beyond 25 watts is not an option.  The 4 dBi antennas might not be a real improvement because they all have the U.S. interference issue.  Even the “ideal” antenna has that issue.  Should I be looking at something different instead that would give gain only at low angles?  Phased verticals or a four square? Loops?  Or a beam on a very tall tower (like 100′)?  Am I being overly concerned about East Coast interference?  Should I just put up a compromise 4 dBi antenna and see how it goes?  Time to take a break and ponder all this.

Ok, times up.  The compromise I am choosing is the inverted vee at 40′.

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