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[Articles Home]  [Add Article]  

Resonant Listening Antennas -- Why?

from Rick Blythe, VE3CNU on April 8, 2013
View comments about this article!

Resonant Listening Antennas -- Why?
By, Rick Blythe, VE3CNU

I had a revelation the other day regarding listening antennas.

As a ham I am very well aware of the importance of SWR and matching antennas to transmitters. Well enough versed in fact to never have worried about it too much. An Elmer I had in the 1970s told me, whatever power is reflected back from your antenna to your transmitter will meet a mismatch there as well, and will just get reflected back to your antenna system, and will be radiated. Don't worry about it he said, unless you're running VHF or higher, or your transmission line is extremely lossy.

I have seen this wisdom repeated by knowledgeable folks here and there and trusted this advice for decades. It has served me well.

Here's the revelation part. Whenever is has come to receiving antennas, I have never given much thought to resonance, matching or anything of the like. I've always had good results from attaching a random length piece of wire to my shortwave receiver and enjoyed good results. Or so I thought. I even wondered to myself why SWLs even bothered to worry about resonance or feed-lines? It only mattered when transmitting in my book. I think I was wrong now.

RF coming into your shack via some feed-line will meet your radio at whatever impedance it presents. Yes, if there is a mis-match, RF energy will be reflected back up the feed-line. Here's the point- Once reflected back to the antenna system, the majority of the energy will be radiated out of the antenna!

I never considered that before. I figured, I'm receiving only, nothing is getting transmitted here. That's a wrong assumption. We might only be talking about micro-volts, but your antenna is transmitting reflected energy!

Think about that!

Member Comments:
This article has expired. No more comments may be added.
 
Resonant Listening Antennas -- Why?  
by G3RZP on April 8, 2013 Mail this to a friend!
Known in the EW sphere for some years - something you have to watch when doing stealth work.
 
Resonant Listening Antennas -- Why?  
by N5NNS on April 8, 2013 Mail this to a friend!
Yes, having built many antennas over 32 years, HF and VHF/UHF, resonance in and of itself can be seen in the received signal strength on HF as well as VHF/UHF. One can see this using the noise level of the radio, GDO or actual signal strength of a known station.

If you wish to see this in action choose a National Weather Service station you can copy on a rig with an S meter. Cut a simple dipole for this frequency and make it a couple of inches longer than needed. Check the signal strength as you trim the wire for the desired length. It will increase as the target frequency is approached.

73, God bless,
George
 
RE: Resonant Listening Antennas -- Why?  
by N5NNS on April 8, 2013 Mail this to a friend!
Or even more simple, listen to a 40 meter signal on 40 meters and switch to your 20 meter antenna or vice versa.
 
RE: Resonant Listening Antennas -- Why?  
by AA4PB on April 8, 2013 Mail this to a friend!
Receiving is all about signal to noise ratio. If most of the noise is generated outside of the receiver (as it is on most HF bands) then matching the antenna system to the receiver increases the signal and the noise by the same amount so you have no change in the signal to noise ratio. Thats why the mismatched random wire works so well on HF receivers and impedance match is not so important in that case.

VHF/UHF is a different story because the majority of the noise is usually generated by the internal receiver circuits. In that case matching the antenna system to the receiver input impedance does make a difference because it increases the signal without causing a corresponding increase in noise so the signal to noise ratio improves and you copy better.

Antenna element resonance is not important - not even for transmit. What is important on transmit is the impedance match. It so happens that a 1/2-wave dipole usually has a feed impedance of around 50 Ohms at resonance so we sometimes mistakenly think that any antenna must be resonant in order to provide a good impedance match and/or good performance. Lets look at the case of a 1/2-wave folded dipole as an example. The feed impedance at resonance is about 300 Ohms. If we feed that with 50 Ohm coax then we'll have an SWR of 6:1 which will cause most transmitters to reduce power output to protect themselves. The antenna is resonant but it doesn't perform very well because its impedance is not matched to the feed line. Now if we add a 6:1 balun at the antenna feed point then the impedance is matched to the coax and all is well.
 
Resonant Listening Antennas -- Why?  
by W5DXP on April 8, 2013 Mail this to a friend!
I wrote a previous article about what an antenna tuner accomplishes looking at conditions from a transmitting standpoint. Exactly the same laws of physics apply to receiving where the antenna is the source and the receiver is the load. In a well-designed low-loss system, when the transmitted signal is tuned for a system match, the receiver will also be tuned for close to a system match. In such a system, tuning for maximum noise on receive gets the antenna tuner in the ballpark of being tuned for a match on transmit.

In a *low-loss* system, when we adjust the antenna tuner for a match during transmit, there will be a 50 ohm Z0-match at the tuner input. If we disconnect the transmitter from the tuner and connect a 50 ohm dummy load across the tuner input, we can look at conditions during receive (assuming the receiver has a 50 ohm input impedance).

If we disconnect the antenna and measure the impedance looking back toward the shack, in a low-loss system, we will measure pretty close to the conjugate of the antenna feedpoint impedance at the transmit frequency, i.e. if the antenna feedpoint impedance is 100-j444 ohms, we will measure close to 100+j444 ohms looking back down the feedline toward the shack. It is not exactly the conjugate because of feedline/tuner losses which are unavoidable in the real world. Let's take a look at what real-world losses do to an ideal conjugate match:

Assume that antenna feedpoint impedance of 100-j444 ohms. A 16.82 foot piece of Wireman #551 ladder-line will result in a resonant impedance value, looking into the ladder-line, of 45.76 ohms. Doubling the length of the ladder-line to 2x16.82 feet yields an impedance of 107.4+j442.7 ohms. In a perfect world with a lossless transmission line, that value would be 100+j444 ohms. So our real-world "conjugate match" is off by about 0.1% in impedance magnitude - close enough for amateur results.:)

For a real-world *low-loss* antenna system, when we adjust the antenna tuner "to make the transmitter happy", we are also "making the receiver happy", i.e. we are coming close to achieving a system wide match. And a system that is "flat" for transmitting, i.e. an SWR of 1:1 on 50 ohm coaxial feedline, will also be "flat" for receiving. In both flat cases, the source and the load are 50 ohms fed through Z0=50 ohm coax.

A test to see how well a system is tuned for receive would be to tune the system for transmit, switch to receive, and try to increase the S-meter reading by adjusting the tuner knobs. In a well-designed low-loss system, there will not be much difference.

If there is much loss in the antenna feedline/tuner system and/or if the receiver impedance is not 50 ohms, then the above power transfer reciprocity will not hold but every system still obeys the same laws of physics.
--
73, Cecil, w5dxp.com
 
RE: Resonant Listening Antennas -- Why?  
by AA4PB on April 8, 2013 Mail this to a friend!
"If there is much loss in the antenna feedline/tuner system and/or if the receiver impedance is not 50 ohms, then the above power transfer reciprocity will not hold but every system still obeys the same laws of physics."
--

True - but the antenna system loss on most HF bands decreases the signal and noise strength equally so we don't notice the effects of a reasonable amount of loss on receive.
 
Resonant Listening Antennas -- Why?  
by K0JEG on April 8, 2013 Mail this to a friend!
Loop antennas are usually course tuned by listening for maximum signal or noise at the desired transmitting frequency. Once you get the hang of it, you can usually get under 1.5:1 just by listening and watching the s-meter.
 
Resonant Listening Antennas -- Why?  
by K1DA on April 8, 2013 Mail this to a friend!
High quality communications receivers are 50 ohm input devices, unless they have an additional LF long wire antenna terminal. If the RECEIVER doesn't see 50 ohms, signal transfer is reduced. I don't think it matters much what the antenna looks like so long as the receiver input is matched.
 
Resonant Listening Antennas -- Why?  
by N4UFO on April 8, 2013 Mail this to a friend!
The word of the day here is EFFICIENCY. When you work with narrow banded, reduced size antennas, you will pick up on this quite quickly. I don't have to look outside to know if it rained overnight or if there is a heavy dew. All I have to do is turn on 12m meters and listen. If it is really quiet, I know my antenna is wet and the resonance has shifted out of band. (All antennas shift when they get wet, even a dipole; reduced size antennas are just more affected by it. And my antenna is most sensitive to this on 12m.)

As for the argument that both noise and signal are lowered... sorry. That might be, but in my world... a weak signal is a weak signal. If you lower the noise and signal, there are going to be some signals lost below the threshold of reception. No way around that. An efficient antenna is always going to hear and 'talk' better than a random one, given equal relative size, height, etc. THEN you utilize matching elements to come up with a proper load for the radio. Whether that be done (preferably) at the feedpoint or at the load end, matching is just that... matching. Matching does not cure the woes of a poorly designed antenna.

In other words, you can't put a high speed transmission from a Ferrari into a go kart and expect it to go 100 MPH... not gonna happen. Just because it has the proper ratio of gears to to let a Ferrari achieve those speeds, does not mean it will do the same for a go kart. And just because it is usually a negligent difference for the average hams ear, does not make it absent. (In other words, most people never hear what they aren't hearing... so they never KNOW!)

This is right up there with putting a preamp just before the rig instead of at the antenna and then blaming the preamp. =^D

Cheers, all! Good one to get the brain & juices flowing first thing in the morning...

 
Resonant Listening Antennas -- Why?  
by K5SBR on April 8, 2013 Mail this to a friend!
Matching the receiver to the antenna, a la W5DXP's explanation, is very, very ctitical for modern receivers using low-impedance bandpass filtering. The results are easily observed. However, my old HQ-129-X doesn't mind so much...as long as the "Antenna Trimmer" control still has an effect.
 
RE: Resonant Listening Antennas -- Why?  
by AA4PB on April 8, 2013 Mail this to a friend!
If the signal is below the noise then you aren't going to hear it. If the antenna system has so much loss that the weak signals drop out then the noise will also drop out. If when you connect your antenna the noise level goes up then you aren't going to have any signals dropping below the threshold of detection (at least not any that you could copy above the noise anyway).
 
Resonant Listening Antennas -- Why?  
by W4VR on April 8, 2013 Mail this to a friend!
When noise is a problem I find that using an 80 meter dipole on 160 for receiving does a much better job than my inverted L. Some years ago I used an 80 meter dipole at 6 feet above ground for receiving on 160, as I did not have room for a beverage. At my current location I use a 300-foot 2-wire switchable beverage that is pretty well matched on all bands.
 
RE: Resonant Listening Antennas -- Why?  
by W5DXP on April 8, 2013 Mail this to a friend!
> AA4PB wrote: ... the antenna system loss on most HF bands decreases the signal and noise strength equally so we don't notice the effects of a reasonable amount of loss on receive. <

Absolutely true for far-field/sky-wave noise. But a noise level is often fixed and local. Under those circumstances, getting the antenna system to deliver the most power to the receiver is a definite advantage. 60 dB of dynamic range doesn't help very much if the local noise level is fixed at S7.

I was just trying to point out that if the transceiver's transmitter (designed to drive 50 ohms) is tuned to near maximum power transfer, then the transceiver's 50 ohm receiver is most likely tuned fairly close to maximum power transfer in the opposite direction (for low-loss antenna systems).
--
73, Cecil, w5dxp.com
 
RE: Resonant Listening Antennas -- Why?  
by AA4PB on April 8, 2013 Mail this to a friend!
Even if the noise source is local and fixed, if it is entering the system only via the antenna then any antenna system loss is going to affect the signal and the noise by the same amount. The only way for the antenna to affect one without the other is to change its directional characteristics.

I do fully agree with your primary point about the impedance match however.
 
RE: Resonant Listening Antennas -- Why?  
by KF5UQJ on April 8, 2013 Mail this to a friend!
So does this mean that I no longer need to drop 90 bucks on a decent SWR Meter? I can just tune my homebrew antenna by listening? :)

Alan (new ham)
 
RE: Resonant Listening Antennas -- Why?  
by AA4PB on April 8, 2013 Mail this to a friend!
Well, if you have a tuner in the shack you can adjust it for maximum signal or noise in order to get fairly close. You'll likely still need an SWR meter in order to make final, more accurate adjustments for transmit.

It would be rather difficult to acutally trim a dipole by adjusting it for maximum signal or noise just because you wouldn't be able to take intantanious readings. There would be a time lag between the time you trimmed the antenna and the time you could make another signal or noise reading. Signal and noise levels are often variable, depending on propogation so you need to see the affect of antenna changes in very short time.
 
RE: Resonant Listening Antennas -- Why?  
by W5DXP on April 8, 2013 Mail this to a friend!
> AA4PB wrote: Even if the noise source is local and fixed, if it is entering the system only via the antenna then any antenna system loss is going to affect the signal and the noise by the same amount. <

The velocity and attenuation factors are not the same for local common-mode noise vs differential mode signals.
--
73, Cecil, w5dxp.com
 
Resonant Listening Antennas -- Why?  
by N0AH on April 8, 2013 Mail this to a friend!
Most designed RX only antennas listen for information contained within the magnetic portion of the electromagnetic wave. Since they both contain the same information, the magnetic portion of the wave is much weaker, thus needing a pre-amp to boost the reception while minimizing noise contained within the electroportion of the wave. This is the theory behind a magnetic loop broadbanded (1.8-30MHz) RX antenna. When mounted vertically, it is even further enhanced by being directional. Pixel RX magnetic loop antennas comes to mind........as do beverages- So the best RX antennas to eliminate noise, while maximizing reception, are broadbanded, and not always resonant for TX. They instead listen for the information contained within the magnetic portion of thewave using designed preamps to help boast enough signal to retreve information while minimizing noise-
 
RE: Resonant Listening Antennas -- Why?  
by G3RZP on April 9, 2013 Mail this to a friend!
But for lowest noise, you don't match the rx to the antenna - a matched system has a noise figure of 3dB. Optimum source impedance rather than a match is required for really low noise figures. At HF it rarely makes much difference. After all, even with 50 ohm input filters, the source impedance is not 50 ohms once you get out of the band.

For Electronic Warfare, where stealth is needed, a good antenna match minimises re-radiation.
 
RE: Resonant Listening Antennas -- Why?  
by G8HQP on April 9, 2013 Mail this to a friend!
A perfectly matched receive antenna still radiates half of the received energy. A poorly matched antenna may radiate more or less, depending on the impedance ratios.

I don't know if this has ever been used, but a form of stealth communication might be possible by merely changing the impedance seen by an antenna and so varying the reflected power. Range might be poor!
 
RE: Resonant Listening Antennas -- Why?  
by N6AJR on April 9, 2013 Mail this to a friend!
Remember the FT 101 series of radios, and others of the same time period. you always "tuned" the plate and load and preset and I forget what else to the loudest hiss. Tthis got you in the ball park for the transmitter side.

BUT when you did this, you could also hear better. It may not be reciproical, but It does make a diference. hook up a manual tuner to the recieve side of your radio and tune for loudest signal, then switch back to the plain input. you will hear the difference.
 
RE: Resonant Listening Antennas -- Why?  
by KM3K on April 9, 2013 Mail this to a friend!
AA4PB wrote: "The antenna is resonant but it doesn't perform very well because its impedance is not matched to the feed line.".
Perhaps AA4PB actually meant to write "The antenna is resonant but the overall-antenna-system doesn't perform very well because its impedance is not matched to the feed line."
The sentence as originally presented by AA4PB would be heresy in view of the writings of Walt-W2DU in his book "Reflections III", specifically page 2-3 #5; W2DU uses as his reference none other than Grammer-W1DF.
73 Jerry KM3K
 
RE: Resonant Listening Antennas -- Why?  
by AF6AU on April 9, 2013 Mail this to a friend!
A matched antenna has better signal strength to the receiver verses unmatched, if the antennas are about the same size. A larger or gain antenna with a 3:1 match can outperform a perfectly matched ground plane.

The transmitted RF and atmospheric noise hits the antenna, and induces an electrical signal. The rig receives the desired electrical signal and atmosphere noise signal, but the reciver also makes internal noise. If the total of all the noise exceeds the desired signal, you hear noise.

A poor antenna receiving a weak signal, may not produce a usable signal from the receiver, if the receivers' internal noise washes it out. A good antenna can receive the same signal, and get a slightly stronger received RF signal to the receiver, one that exceeds the radio's noise floor, and you hear it.

Yep, with weak signals, the stronger the signal to the receiver, the better off you are. Use a gain antenna, and you improve signal to noise. Low loss cables also improve signal. The more signal to the recvier, and you improve the overall signal to noise.
Get the antenna as high as possible, as noise free as possible, match everything the best possible, and get that weak rare DX into the receiver front end with minimal loss.

Bottom line.
You can't work them if you cannot hear them.
JML AF6AU
 
RE: Resonant Listening Antennas -- Why?  
by K9MHZ on April 9, 2013 Mail this to a friend!
>>>>by N6AJR Remember the FT 101 series of radios, and others of the same time period. you always "tuned" the plate and load and preset and I forget what else to the loudest hiss. This got you in the ball park for the transmitter side.
BUT when you did this, you could also hear better<<<<


Yeah, I don't know if the 101 was the same or similar, but on the Tempo One (FT-200), the "grid" control also acted as a receiver preselector of sorts. If you had it set for max receive signal, when you went to add a little transmit drive to adjust the grid matching from the diver plate, you were always very close.

Wow, memories!

 
RE: Resonant Listening Antennas -- Why?  
by N4JTE on April 9, 2013 Mail this to a friend!
Cool, glad the obvious became obvious, enjoy your lisening.
Bob
 
RE: Resonant Listening Antennas -- Why?  
by G3RZP on April 10, 2013 Mail this to a friend!

G8HQP said:

>I don't know if this has ever been used, but a form of stealth communication might be possible by merely changing the impedance seen by an antenna and so varying the reflected power.<

One form of passive RFID tag does this.
 
RE: Resonant Listening Antennas -- Why?  
by K5OX on April 10, 2013 Mail this to a friend!
This is something my supervisor corrected me on, without reservation, soon after I landed my first real job when I was 19 years old, and of course I knew everything!

We were discussing VSWR and I chipped in about how important it was. My boss had a strange look. And instinct held my tongue.

A few discussions later I found out that his background was in receiver front end design. He wasn't a ham and noise figure was his view of the world althouh the two certainly are not indepedent of each other.

It is not always great to be old, but sure is nice to know everything.

Good luck!
Frank

 
Resonant Listening Antennas -- Why?  
by W3TTT on April 10, 2013 Mail this to a friend!
Yes, a resonant receiving antenna will present a higher signal voltage at the receiver, than a non-resonant antenna. But modern receivers have nearly infinite amplification, so as long as the signal level is above the noise, the signal will be readable.
 
RE: Resonant Listening Antennas -- Why?  
by W8AAZ on April 10, 2013 Mail this to a friend!
Don't forget other variables such as height, polarization, and directionality. And ground quality. Any of those could result in a non res. antenna out performing an ideal matched one?
 
RE: Resonant Listening Antennas -- Why?  
by W5DXP on April 11, 2013 Mail this to a friend!
We need to remember the difference between the "antenna" that starts at the antenna feedpoint (load) and the "antenna system" that interfaces to the transmitter (source).

When we connect an antenna analyzer to a properly tuned antenna system and vary the frequency, we see the effects of resonance at a certain frequency (based on the IEEE definition of resonance) where the resistance has some real value, e.g. 50 ohms, and the reactance equals zero. On each side of that resonant point, the reactance goes up indicating non-resonance. Most people agree that the following "flat" system is resonant.

XMTR-----50 ohm coax------50 ohm 1/2WL dipole

Of course, there's nothing magic about 50 ohms. Resonance simply means that the reactance is zero at some frequency and increases on each side of that resonant frequency.

Let's take an example of a clearly non-resonant antenna, a 1.25 wavelength EDZ, extended double zepp, with a feedpoint impedance of 189-j1000 ohms. The SWR on the Z0=600 ohm open-wire feedline is ~12:1. Using VK1OD's transmission line calculator, 11.478 feet of Z0=600 ohm open-wire feedline will result in an impedance looking into the feedline of 50+j0 ohms for a perfect match to a 50 ohm transmitter (through a 1:1 choke).

XMTR---1:1 choke----11.478' 600 ohm line----EDZ dipole

This looks like a very low loss resonant antenna *system* to me in spite of the fact that the EDZ dipole is non-resonant and there's an SWR of 12:1 on the open-wire feedline. I would contend that the non-resonant EDZ dipole radiates just as efficiently as the flat system above AND has about 3dBD gain to boot. In fact, I would also say that when we connect 50 ohm coax to the above EDZ *system*, we have a flat antenna *system* with an SWR on the coax of 1:1.

If we replace the XMTR with a 50 ohm dummy load, we can take a grid dip meter and detect system resonance at any point in the system including the ends of the EDZ dipole - indicating system-wide resonance.
--
73, Cecil, w5dxp.com
 
RE: Resonant Listening Antennas -- Why?  
by G8JNJ on April 11, 2013 Mail this to a friend!
G8HQP said:

>I don't know if this has ever been used, but a form of stealth communication might be possible by merely changing the impedance seen by an antenna and so varying the reflected power.<

Yes I remember trying this in my teeens on 27MHz. An ex-telephone carbon mike insert connected across a dipole element acting as a reflector. Produced some low level modulation when I shouted.

Then of course there was the 'Thing'

http://en.wikipedia.org/wiki/Thing_(listening_device)

Regards,

Martin - G8JNJ

www.g8jnj.webs.com
 
RE: Resonant Listening Antennas -- Why?  
by NZ5L on April 13, 2013 Mail this to a friend!
Sorry, but AA4PB and W5DXP are correct - assuming a reasonable match to the impedance of the receiver, there is no advantage to be gained from a "resonant" length of antenna wire for improving the signal to noise ratio. This kind of thinking comes from considering the antenna and transmission line as separate entities, rather than as components of a complete system.
Some of the best reception I have managed on 160Mtrs has been with loops (FAR shorter than a wavelength) tuned to "resonance" with a homebrew matchbox in the shack. The loop had such a high "Q" factor that most noise was rejected, so a preamp actually helped the S/N ratio. That's the name of the game for low band op. and whatever helps that moves you closer to DXCC or WAS. It might make sense to have a yagi beam to aid your S/N, but even here most guys don't insist on full size "resonant" elements - if the matching components are built into the antenna itself, it will act like a "resonant" system. but without those all-important traps it's just some random length aluminum tubing. And below 40 Meters, it's not even practical from an engineering standpoint.
Without a precision test setup, it's easy to make subjective judgements, but it doesn't change the physics involved.
If we should get the proposed 600Meter band, I doubt if you will see many full sized verticals or dipoles. Still, whatever makes you happy.............
 
Resonant Listening Antennas -- Why?  
by K7PHX on April 19, 2013 Mail this to a friend!
Rick,
I think you can continue to use a random length of wire without any worries.

You have just grabbed a microscopic amount of the total rf field expanding outward from a given transmitting antenna. Some of that energy will arrive at the receiver's rf amplifier and some will be reflected - an even smaller amount than what was extracted from "space".

Assuming for the moment that the antenna "re-transmits" the rf it just received, the signal will be of even lower amplitude that what was extracted in the first place. You would be in one sense "returning" a part of the original rf energy your antenna originally picked up. You are not creating a new signal, or interfering with the the original.

The real advantage of a resonant and matched antenna system is gain and selectivity at the receiver. Of more interest to stealth operation is the transmitting of the local oscillator signal in the receiver.
 
Resonant Listening Antennas -- Why?  
by WA1UFO on April 23, 2013 Mail this to a friend!
I think that is correct. I still get good results with a random wire outside but it works best with a preselector, passive or active. I have a mfj 1020c which is great and affordable.
 
RE: Resonant Listening Antennas -- Why?  
by N1DVJ on April 24, 2013 Mail this to a friend!
Geesh.... Any 1st year EE student has it drilled into their head that maximum power transfer occurs when the impedances match. Come on guys! It doesn't have to only be hundreds of watts going out. It can also be fractions of a watt coming in.

When I had an external tuner, I ALWAYS tuned up for best receive before I started my 'fine tuning' for transmit.
 
RE: Resonant Listening Antennas -- Why?  
by W5DXP on April 25, 2013 Mail this to a friend!
> N1DVJ wrote: Any 1st year EE student has it drilled into their head that maximum power transfer occurs when the impedances match. <

If we have a source impedance of 100+j100 and a load impedance of 100+j100 ohms, the "impedances match" but that is not the condition for maximum power transfer which would occur with a load of 100-j100 ohms yielding a conjugate match.
--
73, Cecil, w5dxp.com
 
RE: Resonant Listening Antennas -- Why?  
by AK4OL on May 1, 2013 Mail this to a friend!
K7PHX: "You are not creating a new signal, or interfering with the the original. "

But you are. This signal constructively or destructively interferes with the original signal and ultimately affects the world around you. Not necessarily a lot more than other conductive objects like houses, trees, vent pipes, power lines, towers, and other antennas but it can be when you are near resonance. Your receive antenna becomes part of the multipath clutter and its effect varies depending on the impedence mismatch. If the impedence mismatch varies with time, some very interesting things can happen.

RADAR scrambler: a tuned antenna at the frequency used by police RADAR was audio frequency modulated by shorting it to ground through a transistor. Arguably, you weren't legally operating a transmitter but rather providing a dynamic load. The tuned antenna was reportedly a better reflector than the car and the speed read on radar was a function of the audio modulation frequency.

RFID tags: some or all RFID tags work by modulating the load seen by the antenna (near field). It has also been demonstrated that one specially designed RFID tag can passively talk to another tag by modulating the radio waves emitted from the reader.

Psuedo Doppler Radio Direction Finding side effects: If you locate a psuedo-doppler direction finder at a shared transmitter site like a mountain top with repeaters and transmitters for other services, bad things are supposed to happen. The pseudo-doppler antenna, with its varying load, inadvertently modulates other signals such as cell phone, FM broadcast, TV broadcast, police dispatch, etc. Also, the impedence changes on each antenna in the set can affect the other antennas. It makes a difference whether or not the 3 or more unused antennas at any instant are open, shorted, or 50 ohm (or impedence matched). Open may be better for receiving a doppler but 50ohm may be better for reducing the effects on the environment.

Try changing the impedence seen by the reflector element on a simple yagi antenna. Set it up to jumper between shorted (normal), open, and characteristic impedence. The whole point of that element is that the reflected signal does matter.

It has been demonstrated that you can make a radar transponder using passively modulated retroreflective antennas.

If you aim your foxhunt RDF antenna at the "reflecting" antenna from a location which is shadowed with regard to the original signal, you may get a reading on the nearby antenna instead of the original source. It isn't unusual for a reflected signal to be stronger than the "direct" signal (from a 2d perspective" because the "direct" signal has to diffract around rooftops, hilltops, or treetops.


As to the original question, if you are in the sahara dessert and you only have 8oz of water, do you want to spill 90% of it? It is one thing to spill it when you have plenty to spare. You have receiver noise figure, you have nearby noise sources that are not necessarily picked up by the antenna (such as switching power supplies on HF), and you have noise sources at different vertical angles (as well as horizontal ones) including galactic, atmospheric, other stations, and other man made interfeence. One of the tricks of NVIS is to put the antenna low so you not only deliberately have a skywarmer but you also are blocking all the noise sources (including DX stations and lightning) near the horizon - a compromise antenna at medium height may be adversely affected by noise. If you are working DX you don't want interference from the NVIS signals and if you are working NVIS you don't want interference from DX/lightning. This is probably why the antenna 8ft of the deck worked better than some others. Since some noise sources are not coming through the antenna, not all noise goes down proportionally to signal. And an input impedence mismatch may actually change your noise figure. If the receiver sees 1000 ohm instead of 50ohm, that might be similar to hooking a 1000ohm resistor instead of 50ohm resistor during noise tests - you might be looking at 4.472 times as many RMS nanovolts of johnson noise per root Hz at the same time that the signal is being reduced by the mismatch.

Receiver noise is generally less than received noise at HF but between 15-30Mhz, receiver noise starts to dominate if you aren't at a noisy site.
http://www.ab4oj.com/icom/nf.html

 
Resonant Listening Antennas -- Why?  
by VE3XQQ on May 8, 2013 Mail this to a friend!
I use a resonant antenna in the format of a modified K9AY loop that sits on wooden fence post in my residential lot. The resonance is set by the load resistor and the loop size. This gives me a pattern like a beach ball cut in half with 25db front to back ratio. This feeds my ANC4 as noise sense antenna and works quite well at removing local flat panel TV noises from S9 to S2.

If it were none resonant it would be none directional and not as discerning as to the phase angle of the noise.

Resonance can be narrow or broad in terms of bandwidth, the T2FD is broad in resonance and a yagi is comparatively narrow. For me resonance is good for receive, choose the right hammer for the job and bash away.
 
Resonant Listening Antennas -- Why?  
by KV4JT on May 11, 2013 Mail this to a friend!
I have been developing solid state conversions to Hammarlund and Hallicrafters receivers for about three years. I have successfully converted HQ-100, 110, 145 and 170 Hammarlunds and S-85, SX-99 and SX-100 Hallicrafters to 100% solid state. I own about 27 of these radios. I was told by many other hams that all I needed was a long wire antenna for good reception. This is a complete misconception. All of these radios have much better signal when using a resonant antenna. A long wire antenna is usually better when compared to a short wire antenna but a resonant antenna will ALWAYS work better. By the way any tube receiver can be converted to solid state if you know how. I will publish my findings and diagrams whenever I have completed my work. The advantages are enormous.
 
RE: Resonant Listening Antennas -- Why?  
by N1DVJ on May 12, 2013 Mail this to a friend!
Years ago National Semiconductor had a book titled The Audio Handbook. If I'm not mistaken, it had a chapter on radio, mostly for automotive applications. It had a very good section of antennas and resonance and why. It also had something I remember about capacitive antennas having more gain than inductive antennas, but I think that may have been applicable to automobiles because the antenna was so short compared to the desires frequency.

As to tubes vs solid state... Didn't RCA have 'solid state tubes' years ago? Basically, cylinders that plugged in and acted like tube equivalent, and with loading resistors for the filament.

 
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