How to load a quarter wavelength monopole

Can you use an attenuator within the equipment or in-line with the cable ?

If it needs to be just with the antenna, then can you use a poor material for the element ? Given the skin depth at 1GHz, it should be a simple matter to drop a few dB from theoretical perfection.

Perhaps you could write down your calculations for the 50% efficiency case. This would give us a better view. I assume the antenna is going to be resonant at 1GHz rather than exactly a quarter of a wavelength long. It seems that you are then putting the resistor in series with the antenna. I would guess the resonant monopole would be about 30 -33 ohms pure resistance without much loss. Adding 18 ohms in series gives a radiation efficiency around 65% doesn’t it? It also matches quite well to a 50 ohm system so the reflection "efficiency" is pretty good.

Please tell us some more about why you are trying to reduce the efficiency. The most obvious way is reducing the length of the antenna!

Normally you would mount the resistors (or better still a coaxial attenuator) between the coax feeder and the SMA connector. This will provide impedance matching between the antenna and coax giving you more accurate results overall. The configuration is that of a PI or TEE attenuator.

If you are going to build the attenuator yourself and assuming it is low power, use preferrably surface mount resistors, or of otherwise metal film resistors with VERY SHORT leads.

What is the goal of the reduced efficiency? Are you just trying to limit the range of the system? If so another method of "detunning" or reducing an antenna's overall gain is by using resistive paints or lossy dielectric material(i.e. absorber material). Some of these materilas can be obtained in "epoxy" form. The monopole element can be wrapped or burried in these materials.
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