Impedance matching loop antenna - Page 1
Forum section: electronics / rf-microwave

author: petergriffin56
author_rank: Contributor
timestamp: March 21, 2020, 02:36:57 pm
content: I want to use a tuned loop antenna with my RTL-SDR. From what I understand at the point of resonance, only the real (resistive) impedance remains. This should be a few ohms for the resistance of the wire, capacitor and ferrite losses.The input pins of the RTL-SDR are about 3300 ohms, how do I go about matching these two ?I was thinking of a negative feedback amplifier, I've seen some examples of this online but it is generally used for broadband matching when there is no capacitor. Can this also be used for a tuned antenna ?Here is an example of an amplifier used in this way, the rest of the circuit is irrelevant. I won't post a 1000 pictures but a google of "broadband magnetic antenna schematic" shows this is a recurring method.

author: bob91343
author_rank: Super Contributor
timestamp: March 21, 2020, 08:56:36 pm
content: Quote:I want to use a tuned loop antenna with my RTL-SDR. From what I understand at the point of resonance, only the real (resistive) impedance remains. This should be a few ohms for the resistance of the wire, capacitor and ferrite losses.Not true. There is the radiation resistance you need to consider. The calculation you made is for determining the losses, not the impedance. In fact, you try to make the losses as small as you can in relation to the radiation resistance.With a VNA you can see how the impedance varies with frequency and thus decide how to match it to your system. (Get one of the nanoVNA units, very cheap and useful.)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2020, 11:11:56 pm
content: Noise floor probably doesn't matter very much, this is going to be LW I suppose? You can get away with much poorer things than that. Also, op-amps aren't especially noisy in the 1kHz to 1MHz range, among the better ones.Which further suggests that tuning a loop at all isn't a very high priority, and you might as well take the convenience of full instantaneous bandwidth instead.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: March 22, 2020, 02:24:04 am
content: usually you would want to tune a loop antenna if you need to reject a interference frequency (but Q is poor with just a LC), or if you want to transmit efficiently.For receiving, consider a passive loop antenna instead.. and then you can put a filter on it afterwards, even active if frequency is low enoughSo long the amplifier is not saturating the amplifier and the amplifier is not mixing the interference to harmful levels, active is the way to go for receiving IMO.since the system you have is small and light, try active first to see if there is a problem before you beef it up.Actually not sure how much better a passive preselector/tuned antenna is vs an active amplifier as soon as you get past a simple LC topology, since all the L's you add, if not air cored, will add some kind of distortion anyway (and really bad if its saturating)... wheras if you use active filters you might be able to get by with just R's and C's. The first C is actually kind of penalty free, but it restricts bandwidth and requires tuning and mechanical maintenance if its tunable.Really good tuners have multiple circuit elements that are adjustable connected to a complicated, precise, expensive and non mechanically robust system of cams and gears on a long shaft (that rages people when they try to fix it). Or, expensive preselectors that are switched in by bandwidth and are not infinitely adjustable. For microwaves the YIG is the best, since its controlled nicely by just a current, but low frequency ones are kinda complicated.

author: petergriffin56
author_rank: Contributor
timestamp: March 22, 2020, 11:46:43 am
content: Thanks for all the replies !I am hesitant to use a broadband matching op-amp for a few reasons :- The signal I'm trying to detect is a 100KHz CW, no modulation, so a very narrow bandwidth is sufficient.- This signal is also pretty weak, and in the VLF range I'm afraid the 8 bit ADC of the RTL-SDR will be overloaded.Wouldn't a broadband match followed by an active filter, or other methods, have more loss in the end due to all the extra components ? Also the complexity that comes with this method might be a lot for me.So an active receiver would be antenna --> op amp --> filter --> ADC ?

author: coppercone2
author_rank: Super Contributor
timestamp: March 22, 2020, 03:55:03 pm
content: if its VLF you might get better results from a big ferrite rod antenna. This is what I did for my VLF radio, with a tuning cap across some silver mica arrangement to give it a little bit of span adjustment.. and you can add a electrostatic shield tooI glued alot of ferrite rods together with a threaded rod in the middle.

author: radiolistener
author_rank: Super Contributor
timestamp: March 24, 2020, 01:59:41 am
content: Quote from: petergriffin56 on March 22, 2020, 11:46:43 amin such case you're needs magnetic loop with high Q. High Q means very sharp tune. But also it means Q times voltage amplification. And at the same time high Q magnetic loop works as a bandpass filter, so you can go with 8 bit ADCFor high Q magnetic loop you're needs to use very low loss conductor and capacitor. Vacuum capacitor will be a good choice. You can use thick copper pipe as a conductor. Maybe it makes sense to use silver plating for copper pipe. Take care of very good contacts, use good soldering, because it is very important for high Q. Even very small resistance is not acceptable.Quote from: petergriffin56 on March 21, 2020, 02:36:57 pmbroadband matching is not suitable for you, because it will give you low sensitivity and low selectivity. To get better CW reception, you're needs to use high Q tuneable antenna. It means that you're needs to use very high quality capacitor with low loss. Vacuum capacitor is preferable.

author: Georg - PY5ZSE
author_rank: Contributor
timestamp: March 24, 2020, 05:08:35 am
content: The (untuned) loop antenna can be modeled by a voltage source Uo, the radiation resistance Rr, the loop's conductor loss resistance Rl and the loop's inductance L in series. for a 1m loop at 100 kHz values for these are aproximately:Ro = 0,24 mOhm!R1 = 0,1 .. 1 Ohm, dependent on the loop's material and sectionL = 4 µH, Xl = 0,4 Ohm.The voltage Uo is calculated from the receved power as U0 = sqrt(Po * Rr).The noise voltage is calculated as usual as Un = sqrt (4*k*T*Rl*B) (the radiatoin resistance does not generate thermal noise!)As can be seen from these values, the radiation resistance (and with it the received voltage) is extremely low, and the voltage source has an impedance much higher than it's source resistance in series. Also, that the loss resistance must be kept as low as possible to achieve a reasonable S/N. Usually, copper tubing or similar is used. The amplifier should have a low input imedance, and must not have any resistance (generating additional noise) in series to the loop.Send me a pm with your email address, I have quite a collection of literature about active antennas including magnetic loops which I am happy to send you. This stuff includes some proved amplifier circuits.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: March 24, 2020, 10:46:34 pm
content: I thought the input for a RTL SDR was 75 ohms, some one tell me if I am wrong.The Small Magnetic Loop is coupled by a matching system, usually a small loop or a gamma match to the 75 ohm radio input.You probably need a bigger loop than most that are sold, you can make one out of stiff coax and use the shield as the antenna, just connect the center wire to the shield.You then need a big variable capacitor to tune to the correct frequency. if you are just receiving, the capacitor type is not so important but for transmitting it has to handle a lot of voltage. There are calculators on the net the work OK. You should be able to build a receiving antenna in a few hours.A magnetic loop antenna is really a LC network and can tune very sharply. I use one everyday for HF. Think of it as a pretuner or a sharp bandpass filter on the input of your radio.If you are into VLF, a large loop horizontal might be more practical and work better, these can be tuned also.There are also Rhombic and Beverage antennas that are just for receiving.There is a great article, "The Underestimated Magnetic Loop Antenna" Google this.Wally

author: radiolistener
author_rank: Super Contributor
timestamp: April 02, 2020, 12:25:59 pm
content: Quote from: Wallace Gasiewicz on March 24, 2020, 10:46:34 pmNo, it depends on the frequency. Here is 50 Ohm base VSWR measurement for RTLSDRv3: