“Dipoles are better than verticals.”
If you’ve heard this before you won’t be surprised but I hadn’t and it really shocked me.
For years we had been taught to put up dipoles for stateside contacts and verticals for dx. “They” said dipoles have 2.2 dB gain over isotropic and verticals are better low angle radiators. Both are at best half truths. More accurate thinking would be to put up a vertical only when a dipole can’t be erected at the proper height or length. Below is an explanation of why dipoles should normally always be the first choice.
“The dipole is the basic building block of many antennas. A dipole does NOT have 2.2 dB gain over an isotropic radiator when the dipole is placed over earth. The dipole has about 8.5 dB gain over an isotropic radiator! Always remember this when you see antenna models over earth given in dBi. If the model over earth shows a “gain” of about 8.5 dBi, the model effectively has the same gain as a dipole.” – http://www.w8ji.com/antennas.htm (Tom Rauch)
I “fact-checked” Tom’s comment by using EZNEC to model a dipole one half wavelength above average ground. Gain numbers are in the same ball park as Tom’s considering he was modelling at 145 feet above ground and I was using a more attainable 33 feet. In this case the model does seem to agree with Tom.
For a rigorous explanation see Joel Hallas, W1ZR, in QST, November, 2015 p44, Antenna Gain, Part 1: What Do The Numbers Really Mean?.
Verticals ground mounted over average soil conductivity with adequate radials don’t have any gain. This model shows gain of 0 dBi. In other words you’d be giving up 8db of gain by using a vertical instead of a dipole.
(I’m planning on inserting a model of a quarter wave vertical with ground radials over average ground here as soon as I can find someone with NEC4. NEC2 supposedly can’t model ground interaction very well and that’s half of a vertical antenna)
Next we model the identical antennas but this time we look only for the gain at a take off angle of 15 degrees. We chose 15 degrees because many DXers believe this is the optimum take off angle to work the most countries.
From Jim Brown, K9YC:
Ah, some say – but the vertical doesn’t do nearly as well at the high angles that support short distance propagation. Yes, that’s true – but: 1) Don’t forget inverse square law – field strength falls as the square of the distance, so stations at 800 miles are 6 dB closer than stations at 1,600 miles and 9 dB closer than stations at 2,300 miles! You don’t need as much signal to work those closer stations.
In closing the soil determines how well a vertical will work. The one time verticals will outperform dipoles is if they’re over salt water or on a salt water beach.
40 Meters – Practical Example
As promised above here are the two models using EZNEC v.6.0 (NEC2-based because I gave up on finding someone with a NEC4 version). Comparing a dipole at one half wave height to a quarter wave vertical on 40 meters both over average soil we get the following graphs. The vertical is on the left. The shapes are surprisingly similar but the gain is not.
Here are the same graphs larger so the numbers are easier to read.
The green dot marks the 15 degree take off angle. The gain at 15 degrees is -1.48 dBi on the vertical and 4.92 dBi on the dipole. This is a difference of 6.4 dBi. In other words to get the same signal out requires 4 times the power on a vertical. Fifty watts into a dipole is the equivalent of 200 watts into a vertical.
This gain comes at the price of directivity. A vertical, of course, has a 360 degree omni directional pattern. A dipole has a 78 degree beamwidth. The dipole is the winner if it is pointed in the direction of the DX. If the DX is off to the side of the dipole the vertical will be the winner. Here is the same dipole showing the azimuth pattern at 15 degrees take off angle.
The front to side difference is 13.93 dBi which we’ll round to 14 dBi. Now the vertical that was 6 dBi worse off the front is 8 dBi better off the side. This is a power multiplier of 6.31. It would take 316 watts into a dipole to equal 50 watts into a vertical when the DX is off to the side of a dipole.
What is the conclusion? Both antennas are needed. Two dipoles at right angles would work, too.