The importance (NOT!) of matching receiver impedance

Discussion in 'Ham Radio Discussions' started by KL7AJ, Nov 30, 2009.

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  1. KL7AJ

    KL7AJ Ham Member QRZ Page

    Technical topics on QRZ seem to cycle through periodically. The topic of the week seems to be impedance matching of an antenna system to a receiver. Here's a little primer on the matter. When I worked in ionospheric research, we had to get this right, since we were looking at VERY weak signals.

    1) Gain is cheap!

    Some of us who have been around a while remember when some H.F. receivers were "deaf" on the higher bands. QST and Handbook preamplifier and preselector projects abounded, to take care of the sometimes weak performance of receivers. Those days are LONG gone. ANY modern receiver has far more gain than you will ever need (in many cases, far TOO much gain for good performance!

    2) Modern receiver input impedances only VAGUELY resemble 50 ohms! It is a RARE receiver indeed (and probably non-existent!) that exhibits a 50 ohm input impedance across the H.F. spectrum. At HIPAS Observatory, we measured the input impedance of many high end amateur (and mil-spec) recievers with a $45,000 HP network analyzer. The BEST of these receivers exhibited input impedances ranging from 10 ohms to 150 ohms! (ignoring reactance, which at some frequencies was quite significant!)

    3) It is easy to misinterpret the importance of point #2 above. One might conclude that since receiver inputs are so widely varying, you will gain a lot by using a matching network on your receiver. (Again, in the olden days, there were several published "receiver tuner" articles). However, Point #2 is GREATLY overridden by Point #1. The fact is, receiver input matching doesn't matter, because ANY modern receiver has tons of gain you can throw away. In fact, receiver input matching only begins to be important at UHF frequencies.

    4) Although this point has been elaborated COUNTLESS times, both on QRZ and elsewhere, nobody seems to get it. I will repeat it once more, at the risk of sounding tedious, in capital letters. IF YOU CAN HEAR A DIFFERENCE IN THE NOISE LEVEL OF YOUR RECEIVER BETWEEN AN ANTENNA BEING CONNECTED AND ONE NOT BEING CONNECTED, YOU HAVE ALL THE RECEIVER GAIN YOU CAN USE! This is an indisputable fact. There is no advantage whatsoever to having receiver gain beyond this level.

    5) A receiving antenna is a signal generator. The question comes up often as to how to model a receiving antenna. The correct model for a receiving antenna is a voltage source with a fixed series resistance. This resistance value is the radiation resistance....remember the reciprocity theorem.
    With this in mind, it is important to note that the standing wave ratio on an antenna system may be different in receive than in transmit. Remember SWR is determined strictly by the load impedance. In receive, the load impedance is the receiver input impedance, and your antenna is the transmitter! Ideally, the antenna impedance should be the complex conjugate of the reciever/transmission line input impedance....but again, at H.F. frequencies it doesn't really matter.


    Eric
     
  2. N0SYA

    N0SYA Ham Member QRZ Page

    i see an antenna as more of a transformer, one that transforms the free space energy in the area into the rig

    transformers, more than meets the eye
     
  3. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Your "modern" receiver notation makes the difference. "Modern" receivers are generally designed for an unbalanced relatively low antenna impedance (nominally 50 ohms). Many of the older ("boat anchor") receivers were designed for a balanced, medium impedance (between 300 ohms and 600 ohms) load. Using some sort of matching network with those receivers when the antenna is fed with coaxial cable definitely results in a significant improvement in the usable sensitivity of the receiver.

    Receivers like the Collins 75A-1 are basically deaf when fed directly with coaxial cable. However, when a matching network is used between the coaxial cable fed antenna and the receiver antenna input the apparent sensitivity of the receiver then becomes excellent.

    For the article that appeared in Electric Radio a while back on this go to

    http://k9sth.com/uploads/TV_baluns.pdf

    Glen, K9STH
     
  4. KI6DCB

    KI6DCB Ham Member QRZ Page

    I see antennas as transformers which transform my roofline into an aluminum forest.
     
  5. NA0AA

    NA0AA Ham Member QRZ Page

    Eric:

    Alright, I'll take what you say, but then I have a couple of questions:

    1. When I use my el-cheapo SW receiver with a large external antenna, I had problems with local stations overloading the front end - a small matching unit - coil and cap allowed me to blank out the interfering station...In some way this thing enhances the selectivity.

    2. When I use my random [well, it's not really random, it's 30'...] vertical with the SG-230 coupler, I turn on the 230, and it goes into the 'standby' which is direct fed [no tuner] and when I tune it into the band of interest, the signal strength often jumps by a very large amount - from nearly inaudible to say 57....

    I mean, I agree, I'm not changing the actual size of the antenna, but then what is happening? I suppose the background noise could drop that much.
     
  6. AG3Y

    AG3Y Guest

    WOU, you are adding a selective component to the very front of your receiver. Many modern receivers seem to be capable of receiving from "D.C. to daylight", meaning a very broad spectrum of RF energy. However, to do this, designers often leave out vital components that would improve the operation of the receivers for those few select frequencies that we use for our avocation, namely the ham bands !

    This is really not all that new a problem. I have a set of Kenwood "golddust twins", that harken back to somewhere in the late 50s or early 60s for their manufacturing date. The R599 can easily be overloaded with a broadcast band station that is under a mile away from my house, unless I use an antenna tuner in front of it! The antenna tuner acts like a highly selective component that rejects most RF outside of the desired band I am receiving. Not only is the desired band isolated, but the impedance match between the antenna and the reciever is optomized so that a greater amount of RF signal will be transferred from the antenna to the RF amplifer stage of the receiver. As you have noticed, the signal strength will sometimes increase dramatically ! However, as has been stated in other threads, the actual signal to noise ratio may not be better, except for the rejection of the undesired signals due to the limitations of your receiver's "front end".

    Hope this is clear enough. 73, Jim
     
  7. AI3V

    AI3V Ham Member QRZ Page

    Sure Gain is cheap, but what about noise figure?:)

    Old news to weak signal guys, receivers (read-preamplifier's) are tuned not to 50Ω or whatever impedance you think your receiver system is, but to provide the match that has the best noise figure

    Again, thinking of gain, without thinking of noise figure is only part (less than 1/2) of the study...

    Un true. You have NO way of knowing if the difference is caused by the amplifier breaking into oscillations, or a million other causes.

    A sensitive receiver will have a increase in noise when an antenna is attached.

    A increase in noise DOES NOT mean you have a sensitive receiver!!!!!!!



    Again, when we tune a receiver, we are worried about the best noise figure.

    This INCLUDES the ANTENNA gain/noise temperature, The physical (thermometer) temperature, and the receiver noise figure, and a few dozen other factors.

    You can just about bet, that the best noise figure of an amplifier will not coincide with it's maximum gain, or, with the impedance you think your receiver system is.


    Rege

    I have ignored commenting on the design that goes into receiver selectivity, It is "easy" to design a receiver that is sensitive when hooked up to nothing but a single-signal (signal generator in a lab environment). It is quite another thing to design a receiver that will "hear" KTB noise on a crowded band.
     
  8. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page

    That's very true. Most of the older short wave receivers had an antenna trimmer, a small variable cap across the input circuit that peaked the preselector. The trimmer could make a dramatic difference in received signal strength.

    You could also use it along with the RF gain to help attenuate noise and it worked after a fashion by decreasing front end Q.

    As for Eric's assertion about modern rigs, I agree in theory but matching the input impedance will optimize the power transfer from the antenna.

    On the other hand, a single length of wire and a ground connection has always been an effective receive antenna on most boat anchor radios I have owned. A resonant antenna or a beam is better of course but a single wire looking into the high impedance front end of an old tube receiver works well for MF and HF reception.
     
  9. WA0LYK

    WA0LYK Ham Member QRZ Page

    While gain might be cheap, noise is also. Each and every amplifier you have will introduce noise. While this may/may not be noticeable say at 40m or 80m it can be very distinguishable at 20m and above.

    Broadband design introduces several trade offs, one of which is reduced selectivity. Reduced selectivity means you are amplifying broadband noise also. The elimination of a "preselector" knob on your receiver may make operation more enjoyable by not having to "tune" the preamp to a given frequency but it also means strong signals elsewhere can "overload" the amplifier.
     
  10. AI3V

    AI3V Ham Member QRZ Page

    Exactly!:) Not only does a receiver have to be sensitive enough to pick up a weak signal (easy), but it also has to be selective enough to reject everything else (hard, or at least expen$ive).

    The overload you see is usually caused by either the first mixer, or the first amplifier. in the case of the mixer, you need a certain amount of local oscillator power to mix with all the signals in the passband, either desired, or interference- you need a LO stronger than the sum of all signals + the conversion loss of the mixer.

    This is both expensive, and a powerful LO is difficult to keep inside the receiver to prevent it from jamming other users.

    Similarly, the RF amplifiers all have to have enough power handling capability to remain linear with a strong signal. Unfortunately, amplifiers that make good strong signal amplifiers tend to not be very sensitive.

    In any case, using a preselector to cut down on the interfering signals until they no longer overload the receiver is a cost effective method of improving receiver performance.

    There is a old saying "receiver selectivity belongs as close to the antenna as you can get it".


    Fist off, a 57 signal means nothing:cool: How many microvolts is the signal?:) and what is the ratio of signal+noise/noise?:)

    What you are seeing here is simple impedance matching. Your receiver will have a particular impedance, as will your feedline and antenna.

    The antenna impedance will vary more or less as you change wavelength.

    I.E. a Marconi antenna operated on it's 1/2 wavelength will have a high impedance, perhaps 5000 Ω at the 1/4 wavelength it's about 35Ω. Your tuner just "matches" the antenna to the receiver, just like it matches the transmitter to the antenna!

    ASSUMING the noise at you QTH is constant vs frequency, If you switched to a antenna with a more constant impedance vs wavelength (like a discone), you would not see this noise change effect.

    Rege
     
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