I made a 7 MHz antenna trap. On testing it the cut-off point was 6.687 MHz which is about right but the problem I've got is the SWR was an 8 and wouldn't go down to a resonant SWR.
What have I done wrong?
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$\begingroup$ Hi Chris, if you have a moment, would you mind editing your question to include some construction details about the trap you've built? $\endgroup$– webmarcSep 2 at 16:11
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1$\begingroup$ Is the SWR merely high, or are you not seeing a dip at all? There isn't such a thing as "resonant SWR"; SWR is a measure of the impedance mismatch between a nominal value (e.g. 50 Ω) and the actual impedance seen by the measuring device. You'd get a SWR of 2.0:1 if resonant at 25 + 0j Ω, or non-resonant at 40 + 30j Ω. With a trap, you expect the SWR to fall at resonance, but not necessarily to 1:1 - it might only drop from (say) 15:1 to 8:1, because the trap outputs are open-circuit. $\endgroup$– Simon FarnsworthSep 5 at 13:21
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$\begingroup$ Please provide a schematic of what you are using along with parts tolerances $\endgroup$– AG5CISep 17 at 22:04
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2 Answers
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Firstly, there's some confusion in your question: there is no such thing as a "resonant SWR"; resonance happens when the net reactance in your system at the resonant frequency is 0, and can happen at any resistance value. Impedance at resonance is simply the resistance. SWR (among other interpretations) is a measure of how closely matched the impedance of the system is to the nominal impedance. An antenna system that's resonant at 25 + 0j Ω has the same VSWR as an antenna system that's non-resonant at 40 + 30j Ω; they're both 2.0:1. You'd also get the same VSWR at 100 + 0j Ω.
An ideal trap would be a perfect bandstop filter; it'd have a bandwidth in which it has infinite impedance (=> infinite VSWR), and it'd then have zero impedance (=> infinite VSWR) outside that band. The effect of this with antenna wire on both sides is to have the full length of the wire in use outside the stop band, and to "cut off" the wire on one side of the trap when you're inside the stop band for the filter.
In practice, you can't build this; perfect filters aren't possible. Instead, we use a parallel inductor and capacitor to build a series bandstop filter, and call this an "antenna trap". As a loose circuit, it looks like:
simulate this circuit – Schematic created using CircuitLab
Note that the trap is symmetric, and you change the component values to choose the resonance point; I'll leave the proof as an exercise, but resonance happens when $f = \frac{1}{2\pi\sqrt{LC}}$.
When you measure the trap on its own, with one side left unconnected or shorted to the measurement gear's ground reference, and your measurement gear connected to the other side, you will see a very high VSWR away from resonance (where the filter's series impedance is irrelevant, and you're seeing the impedance you'd see if you replaced the trap with a small piece of wire), and it'll drop at resonance, where you're seeing the impedance of the trap in series with the open circuit or short to ground at the other side of the trap, then climb again as you move away from resonance. However, the dip is not guaranteed to be down to 1.0:1 - it could be to 8.0:1 quite easily. You might, for example, see that VSWR is 10:1 from 3.5 MHz to 6.5 MHz, falls to 8.0:1 at resonance, and then climbs back up to 10:1.
With the far side left unconnected, the resulting circuit you're measuring looks like:
With the far side shorted to ground, you get:
If you terminated the far side of the trap with a 50 Ω dummy load (a pure resistance) connecting the far side to the measurement gear's ground, the trap's behaviour as measured on your SWR meter will change; as you get further from the resonant frequency, the VSWR will get close to 1.0:1, while as you get closer to it, the VSWR will increase to indicate the inherent high impedance the notch filter presents at its resonant frequency. So you might see 1.1:1 at 3.5 MHz and at 10 MHz, and 7.0:1 at resonance.
Measuring with a dummy load makes the circuit you're measuring look like:
And it's worth noting that the nature of a practical trap is that as you go down in frequency from resonance, it acts more and more like a loading coil (an inductor connected in series); if you go above the resonant frequency, it'll act like a capacitor in series. This affects how much wire you put either side of the trap, because at lower frequencies, the trap's inductance will act to shorten the amount of wire needed to get to your desired impedance.
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$\begingroup$ "resonance happens when the net inductance in your system at the resonant frequency is 0". Did you mean reactance instead of inductance? $\endgroup$– Mike Waters ♦Sep 12 at 22:40
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1$\begingroup$ @MikeWaters I did indeed - edited to fix. $\endgroup$ Sep 13 at 13:13
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Chris.
If you want to use this trap to allow an antenna to work on the 7 MHz 40 m ham band as well as say on the 80 m 3.5 MHz band , it seems you have the trap around the wrong way.
A trap acts as a low pass filter that will "chop" off frequencies above the cut off point.
So you need that point to be about 7.6 MHz for 40 m, if it's at 6.687 MHZ you are cutting off the band you are trying to use.
Apart from that, there could be numerous other reasons why the SWR on your antenna is bad, the main reason possibly being that the antenna elements or wires are the incorrect length.
Hope that helps !
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3$\begingroup$ Traps are not coils or low pass filters, they're resonant circuits. In a trapped dipole they're used to chop off the rest of the antenna for a narrow range near their resonant frequency, which is the higher of the two, allowing the middle portion to work as a half wave dipole too. See for example ham.stackexchange.com/a/7638/2988 $\endgroup$– tomnexusJul 9, 2022 at 16:00
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$\begingroup$ @tomnexus Thanks for that, i have modified the answer accordingly. $\endgroup$– AndrewJul 9, 2022 at 23:56
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$\begingroup$ @tomnexus Also, from that answer you mention, this comment "Tuning resonance to be above or below the band does not matter much" - i don't agree with that, when the frequency of operation is near the the resonant frequency of the trap, the trap determines the electrical length of the active part of the element not being chopped off, and that does matter as it determines the resonant frequency of that part of the antenna. What am i missing ? $\endgroup$– AndrewJul 10, 2022 at 2:07
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1$\begingroup$ When the frequency of operation is near the resonant frequency of the trap, the position of the trap determines the electrical length of the element. Whether the trap resonance is above or below the operating frequency at that point is a minor factor. What matters is that the trap is a significant enough impedance bump to cause a near-total reflection. (It does make a slight difference, as Phil alludes to, because the trap constitutes an end-load... but not too much of one.) $\endgroup$ Jul 10, 2022 at 5:19