sort of counter intuitive
from my web site www.iel-rf.com and expanded
Using Attenuators as Loads
Copyright Instrumentation Engineering Limited May 2001
RF loads are usually required when testing RF transmitters. The reason for this is that the transmitter has been designed to work into an impedance that is usually equivalent to 50 ohms resistive. Modern transmitters i.e. solid state Bipolar or Mosfet, will not produce their rated output power unless they see a reasonably good load impedance that approximates the above. A good rule of thumb for estimating forward power from solid state PA’s is-
Forward power is bounded by 1/S and 1/S^2 where S is the SWR number (S:1). Typically the rated output power is held constant by the internal control and bias circuits for SWR values up to 1.3:1 or sometimes as high as 1.5:1 (this problem is caused by the load lines for the active devices in the output shifting). At his point the forward power starts to turn down as SWR increases. Typical antennas present load impedances with SWR values as high as 3.0:1. In such cases the forward power could be as low as 1/9 of the value presented into a good dummy load. The true power is 25% less than the 1/9 value due to the reflected power of the 3.0:1 SWR. (25% of forward power is reflected to the transmitter by a 3.0:1 SWR. Therefore the worst case true power delivered to the load is approximately 11dB below the power delivered to our good dummy load by our transmitter.
Most transmitters will work into a load SWR of up to 3.0:1 without damage. An SWR of 3.0:1 is equivalent to a load return loss of 6dB. A 3dB attenuator also presents a return loss of 6dB if the end away from the transmitter is open or short circuited or shunted by a pure reactance.
Dummy loads unless expensive precision types have typical measured return losses of between 15 and 20dB (SWRs of 1.15:1 and 1.05:1) with cheaper units specified at SWR of 1.2:1 ( a return loss of approx 13.5dB).
From the above it can be seen that an attenuator of greater than 3dB can be used as a load that will not damage the transmitter. However if transmitter output power is to be measured with reasonable accuracy then attenuation values of greater than 8dB should be used which will give a result accurate to 0.12dB.
Examples of attenuators as a load use.
1./ From above. If we connected an attenuator of 4dB in series with the antenna the transmitter would see a return loss of 14dB (made up of 6dB from the antenna and 8dB from the attenuator. The transmitter would then supply full power minus 4dB to the antenna, a theoretical gain of 6dB. However the antenna because of its SWR has a mismatch loss of a further 1.25 dB. We are still ahead by 4.75dB in transmitted power than without the attenuator.
2./ By putting an attenuator onto an isolator (circulator with dump load)
Instead of a load, access to the transmission system can be had to check deviation, carrier frequency, and condition of the antenna / feeder. If the attenuator were 10dB in value this would provide 20dB of return loss to the circulator and reflected energy from the circulator would be measured at one tenth actual value. Advantage of this is that all this can be done with out taking the transmission system out of service.
3./ Checking antenna SWR
When checking antenna SWR (return loss) the antenna should be isolated from the output impedance of the transmitter. This is due to the fact that most commercial land mobile transmitters do not have a 50 ohm output impedance. By putting a 10dB attenuator on the transmitter output the measuring bridge (which is designed for 50 ohm source and load) will measure the actual SWR of the antenna with reasonable accuracy. Without the attenuator it is possible that a good SWR indication is had when in fact the true SWR indicates the antenna or feeder has failed.
4./ Placing an attenuator between RF FM Broadcast exciter and RF power amplifier.
If a low value attenuator ie 1dB – 3dB is placed between exciter and power amplifier both the Exciter output and the power amplifier input see increased return loss helping stabilize power control circuits and adding significantly to RF stability of the power amplifier especially if the power amplifier is working into a high Q output load such as cavities or a multi coupler. Harmonic and spurious energy that is reflected by the output cavities is reflected through the power amplifier to its input and onto the exciter output where again it is reflected back to the power amplifier input giving rise to instability of the power amplifier. This effect is most pronounced with modern wideband solid state power amplifiers operating in anything other than pure class A. Class B, C, D, all exhibit this behaviour while class E,F,G also suffer they also tend to modify their input impedance with drive level.
5./ At frequencies in the HF to VHF range for a single spot frequency rolls of 50 ohm coax can be used as attenuators by using the coax makers attenuation vs frequency charts. High values of attenuation are unlikely at low HF frequencies but useful Pa output protection can be had with RG58 or RG174 the later will have to be water cooled for powers above 100watts @ HF