+++++++++++++++++ Date: Sun, 20 Oct 2002 08:13:53 -0700 From: Bob Nielsen To: Elecraft Subject: Re: [Elecraft] dBd vs dBi Antenna Measurements 101 (short version): The dBi reference is compared to an isotropic radiator in free space, one which has an equal signal intensity in all directions (the pattern is spherical). This is the reference typically used when calculating antenna patterns and gain. Compared to an isotropic reference, a dipole in free space has a directivity of 1.64, or 2.15 dB. The gain is equal to the directivity times the efficiency of the antenna (which is usually ignored). Since true isotropic radiators do not physically exist, measurements are often made with a dipole reference. Knowing the directivity of an antenna compared to a dipole, you add 2.15 dB to get the directivity relative to an isotropic reference. Therefore 3 dBd would be equivalent to 5.15 dBi. However, when measuring gain under other than free-space conditions (or in a suitable anechoic chamber, not very practical at HF), ground and other reflections affect the measurement. Since many companies handle this in different ways, it is often difficult to make an exact comparison. Usually a dipole at the same height is used as a reference, but if the antenna in question has a narrow elevation pattern, the effect of the ground reflection may vary between the antenna being tested and the reference dipole. Screens are typically used on antenna ranges to minimize the effect of reflections, but one can get slightly different results when using different test ranges. It is probably safe to say that no two manufacturers use exactly the same measurement criteria and a few cannot be trusted very much at all, which is why you seldom see gain figures in QST. About 40 years ago, an engineer I worked with said in regard to measurement accuracy, "tenths of a dB don't count." Unfortunately, this still often applies. 73, Bob, N7XY Bainbridge Island, WA On Sun, Oct 20, 2002 at 07:42:26AM -0400, kc4kgu at ENTERZONE.NET wrote: > > Can someone tell me what how these two figures would compare to each > other? > > 3-dBd over a dipole > > 3dBi freespace > > I'm trying to compare antenna specs from two different companies but, as > usual, they can't use the same _exact_ measurement criteria. > > 73 de John - KC4KGU +++++++++++++++++ To: kc4kgu at ENTERZONE.NET, elecraft at mailman.qth.net Date: Sun, 20 Oct 2002 10:49:50 -0700 Subject: Re: [Elecraft] dBd vs dBi From: k6se at juno.com John, KC4KGU wrote: "Can someone tell me what how these two figures would compare to each other? 3-dBd over a dipole 3dBi freespace "I'm trying to compare antenna specs from two different companies but, as usual, they can't use the same _exact_ measurement criteria." ========== The term "dBd" means dB gain compared to a dipole. The term "dBi" means dB gain compared to an isotropic radiator. A dipole has 2.14 dB gain over an isotropic radiator, so a dipole has 2.14 dBi gain and an isotropic radiator has -2.14 dBd gain. IMO, the most meaningful and honest term would be "dBi". All antenna manufacturers should adopt this as the "standard". To be honest, any gain specs given should be the gain in free space because when antenna specs are given over real ground (rather than free space), the amount of gain realized can vary widely depending on the soil conductivity in the locale that the gain was tested at. Gain over real ground will always be greater than gain in free space. Be skeptical of any manufacturer's gain claims they give in their specs. Unscrupulous claims of antenna gain is why ARRL will not publish gain figures in QST ads. Some claims are honest, while many are not. 73, de Earl, K6SE ++++++++++++++++++ Date: Sun, 20 Oct 2002 13:53:07 -0500 From: "George, W5YR" Organization: AT&T WorldNet Service To: k6se at juno.com Cc: kc4kgu at ENTERZONE.NET, elecraft at mailman.qth.net Subject: Re: [Elecraft] dBd vs dBi While I agree with the first part of Earl's statement quoted below, I believe that under certain circumstances, ARRL will in fact publish gain figures, etc. The following is taken from the ARRL website: "Antenna advertisements that specify gain, front-to-back ratio, or beamwidth are acceptable if the antenna has been tested in accordance with EIA Standard RS-329, Part 1, and a statement to this effect appears in the advertisement. Certified test results must be submitted with the ad. "In lieu of RS-329-certified results, the advertiser may advertise performance figures derived from specific antenna modeling programs. To do so, the advertiser should first model the antenna, using either YO (Yagi Optimizer, for Yagis only) or the latest version of the NEC (Numerical Electromagnetics Code) program. See the ARRL Guideline for Modeling Antennas for Advertising Acceptance for answers to frequently asked questions and for sample graphs. The data files used must be submitted to ARRL at the time the advertisement is submitted. "All advertising performance claims derived from antenna modeling must indicate that they are calculated. As a minimum, we expect to see the peak free-space gain (in dBi referenced to isotropic) at a manufacturer-specified frequency. ARRL encourages advertisers to provide even more information to end-users, showing performance measurements or computations over a specified frequency range, rather that at a single frequency. "In addition to free-space performance figures, advertisers may wish to show computed performance over ground. If so, they must use a "typical" ground, with a conductivity of 5 mS/m and a dielectric constant of 13. They must invoke the Sommerfeld/Norton ground model if NEC is used, and the terrain shall be flat ground at antenna heights specified by advertiser. "Advertisers may elect to show a computed front-to-back ratio at a frequency they specify. However, ARRL encourages advertisers to show the worst-case front-to-rear ratio, over a frequency range of their choice, preferably a complete amateur band. "ARRL also encourages advertisers to grant permission to place modeling files on the ARRL BBS for knowledgeable users to try for themselves, to foster greater public confidence in performance claims. As always, the ARRL Laboratory reserves the right to inspect and test an actual antenna to verify modeling claims." I am pleased to see ARRL adopt this enlightened policy regarding antenna ads rather than avoiding the issue altogether. 73/72, George Amateur Radio W5YR - the Yellow Rose of Texas In the 57th year and it just keeps getting better! Fairview, TX 30 mi NE of Dallas in Collin county EM13qe K2 #489 Icom IC-765 #2349 Icom IC-756 PRO #2121 +++++++++++++++++++ To: lrahnz at garlic.com, kc4kgu at ENTERZONE.NET, elecraft at mailman.qth.net Date: Mon, 21 Oct 2002 08:06:12 -0700 Subject: Re: [Elecraft] dBd vs dBi From: k6se at juno.com Logan, KZ6O wrote: "You need to be a little careful of the Force 12 numbers. What they mean is that they have taken into account the effect of ground reflections. Notice the phrase "compared to a full sized dipole at same height". ========== Although I prefer "dBi" and and I prefer "free space" if "dBd" is used, Force 12's use of "dBd" over real ground is still quite fair. For example, a 40-meter dipole in free space has a gain of 2.14 dBi. A 2-el 40-meter Yagi in free space might have 6.45 dBi (4.31 dB more than the dipole). Putting those same two antennas 75' over real ground with "average" soil conductivity yields 7.92 dBi gain for the dipole and 11.46 dBi gain for the Yagi. Here the Yagi is only 3.54 dB better than the dipole !! Therefore, Force 12's choice to to "dBd over real ground" is on the conservative side! On the other hand, nobody has *ever* been able to get 8.81 dBi gain out of a 3-element Yagi, as SteppIR claims (about 8.2 dBi is the max possible). 73, de Earl, K6SE +++++++++++++++++++