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Gain and Efficiency, The Big Myth

by Tom Rauch, W8JI

It may be time to call attention to the fact that gain or efficiency has nothing to do with receiving system performance, so long as the background noise or QRM level is high enough to establish noise floor in the receiver. One of the most common and misleading myths is that any system gain increase, either through decreased feedline loss or reduced matching transformer loss, somehow improves reception. This is NOT true, unless the receiving system is limited by internal noise.

I have examples on my website dispelling some common myths. Receiving ability is tied directly to pattern directivity, not absolute efficiency, signal level, or "gain".

Stacked Beverages

A gain increase in Eznec (or some other modeling program) by adding parallel lossy antennas, without mutual coupling effects, will NOT represent increased receiving performance. One example of this is paralleling a number of lossy Beverages with small spatial separation. Neglecting feedline and coupling losses, when the number of close-spaced Beverages doubles the absolute signal level increases 3dB. This signal level increase, however, is not accompanied by an increase in S/N ratio from the antennas unless spacing is significant compared to the length of each antenna! The reason for this effect is simply explained. Efficiency doubles while the pattern does not change.

Reception is improved only when the ratio of response in the desired direction to the sum of responses in ALL other directions is increased! You can find the correct number by finding average gain of an antenna, and subtracting the absolute gain at the angle and direction of the desired signal. I call this the RDF (receiving directivity factor) of an array (more).

Reduced Feedline/Transformer Loss

A recent article indicates the best transformer designs somehow are related to impedance matching and coupling loss. Practical experience and theory disagree strongly with this contention.

In a receiving system, SWR between the receiver and antenna is unimportant except as it increases effective system energy transfer losses. If the receiver hears a reasonable noise increase from propagated noise, changes in loss or matching will not affect reception. Most modern receivers are in the area of -130dBm sensitivity at narrow bandwidths, and even a modest preamplifier with 4dB NF (very easy to obtain at HF or lower) will allow -123dBm signal power levels to be 10dB out of system noise.

As an example, at my quiet rural location a push-pull high dynamic range amplifier (DX Engineering RPA-1) allows 1.8MHz propagated noise to limit lower threshold with only a NON-impedance matched wire of ten feet connected to the amplifier's low impedance input terminal. Matching would substantially increase signal level, but would NOT improve S/N ratio.

My System Examples

My system consists of around 30 Beverages and two arrays of phased verticals with a total of twelve vertical elements. Most Beverages are around 800 feet long, and are in co-phased pairs spaced 350 feet broadside. The directivity of these Beverages is up around 3dB from single antennas on any frequency where the spacing is 5/8th wavelength or more, although best usable performance centers between 1.8 to 4.0 MHz.

The verticals are 20 feet tall, resonated with an inductor/hat combination, and coupled through an intentional series loss resistance of almost 75 ohms to my 75-ohm feedlines.

Total feedline length is over 2500 feet in these systems, before the signal arrives at any amplifying device. The cables are mixtures of flooded (direct burial) F11 (RG-11) and RG6 CATV cables, as well as some .625" hard line (1000 feet long) for the main trunks.

Despite this long feedline length and the intentional matching loss on the verticals, my system clearly limits on external noise. Why then would anyone worry about any modest (under 5 or 10 dB) transformer loss in a matching transformer? Certainly I don't, and I'm miles from any noise sources.

By far the largest worry is unwanted signal ingress into the feedlines, through common mode currents. In effect the shield can act like part of the antenna system if the cable is not correctly balanced for currents at each end. It isn't a matter of double-shielded cables; it is a matter of eliminating common mode coupling. Even a poorly constructed cable has negligible signal ingress through the shield at HF when the shield and center carry exactly equal and opposite currents. Make a poor shield connection anywhere in the system will cause currents to become unequal or incorrectly phased, and allow even a quad-shielded cable to suffer from unwanted signal ingress (check for more).

Summary

Dedicated DX'ers are well advised to beware the pitfalls of considering absolute signal level important, especially if such an increase comes with a penalty of reduced directivity. Reduced directivity through poor common-mode signal rejection in a feed system may not appear as an increase in absolute noise level. You may not know you have hurt things at all.

With any listening antenna, it is an excellent idea to isolate the antenna as much as possible from the feedline shield. This is true even when the increased isolation results in a slight reduction in system gain or system signal levels.

Also beware of gain increases in modeling programs. A gain increase may not be related to an increase in directivity, and when NOT related to increased directivity (or if your receiver is so poor its internal noise sets the system noise figure) the gain increase is useless.

You can find more details of this at my website.

Good DX'ing,
Tom W8JI

published on DXing.info on July 13, 2003

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