Antenna resonance vs swr.

[kidphc]

So building out my system for the truck. Finally, after a year of laziness I am building a center console (well the front half) to house a FTM400xdr and an Anytone at6666 10/12m radio (code for illegal cb radio for those that don't know). It will be used for 10m work as well.

 

STAY TUNED FOR A BUILDOUT POST.

 

So the antenna is mounted to the hood in front of the a pillar. Using a NMO34 with a 64" whip (10m) and a NMO30 with 64" whip(11m).This is going away. The whip is .100 and it flexes to 45 degrees above 30 mph and much more at 70. Incoming MFJ 10m hamstick. Should be tunable for both bands.

 

I was trying to hold to Marc's talk about every commercial antenna for 11m is too short even with a spring. SIDE NOTE: I DID FIND SOME ONE THAT MAKE CUSTOM EXTENDERS TO GET A 102" TO 115" OR SO.

 

Now back on track. The nmo34/30 combo the lowest I could get was a SWR of 1:1.6 or 1:1.7 with trimming. I gave up because the SWR started climbing back up. Lowest frequency for the 10m set up was 28.594 (VSWR OF 1:1.6). Just a hair away from the target of 28.400.

 

It started making me think of antenna resonance and how to calculate it.

 

So in order to find the resonant frequency of an antenna I am looking for an impedance of about 75 ohms purely resistive? This the value I gather for a dipole in free space.

 

So the question is how the hell do I find resonance of a custom, less then ¹/4 wave whip with a bottom loaded coil?

 

 

 

Sent from my SM-G975U using Tapatalk

[n4gix]

With about $230 to buy a RigExpert antenna analyzer. Seriously:

 

[kidphc]

That is one way. But a rigg expert will have to wait. Was hoping for something like with the nanovna like at lowest reactance and inductance.

 

Actually might have found something on those lines.

 

From: https://www.electronics-notes.com/articles/antennas-propagation/antenna-theory/resonance-bandwidth.php

 

 

Basically, the lowest impedance value.

---

This is the frequency where the capacitive and inductive reactances cancel each other out. At this point the antenna appears purely resistive, the resistance being a combination of the loss resistance and the radiation resistance.

 

Well kinda. For a pre-engiered antenna generally 1.5 swr is where the antenna is resonant. But an antenna can be resonant t frequency and have a really high swr say like 1:5, example folded dipoles.

---

[n4gix]

Actually a nice nanoVNA would certainly work as well. It's a bit more of a learning curve simply because it doesn't have the pre-written macros like a RigExpert has. If you go that way, spend the few dollars more to buy one with the largest screen you can find, as the smaller versions have such a tiny font they are very difficult to read...

 

Truthfully, a nanoVNA can do a lot more than the RigExpert does, certainly more than the MFJ analyzers can do. There's a reason why MFJ is semi-jokingly called "Mighty Fine Junk..." 

[kidphc]

I have a nanovna ver h4.

 

Kinda why I am asking how you would look for the resonance. It certainly has way more ability then the user to do many functions at this point.

 

Sent from my SM-G975U using Tapatalk

[n4gix]

Well, when you run a test for SWR it will tell you the resonant frequencies. Plural because it will likely show at least the first three harmonics if the bandwidth is set widely enough. This YouTube may help.

 

https://www.youtube.com/watch?v=b88f_DEm5rg

[mbrun]

Resonance and SWR are two different things.

 

Resonance speaks to how closely tuned the antenna is the wavelength of the frequency you desire to operate on. SWR speaks to how closely the impedance of the antenna is to the characteristic impedance of your feed-line (e.g. coax). If the antenna is perfectly resonant, its impedance is exactly 50-ohms and the antenna is connected to a good 50-ohm coax, then your SWR will be 1:1.

 

Now, if your antenna is perfectly resonant but it’s impedance is 75 ohms, you will never achieve a real SWR of 1:1 using 50-ohm coax. You cannot achieve it because there is an impedance mismatch between the two. You can trim and lengthen the antenna all you want, but you will never get to a 1:1 unless you add sufficient extra coax to eat up and waste all your reflected power (a waste). Hypothetically though, switch to a radio designed for 75-ohms, use 75 ohm coax and you are back in business, 1:1.

 

A perfectly resonant antenna will absorb (i.e. radiate) all of the power you send it using the frequencies for which it is resonant, assuming off course you send it from a source and over feed-line that matches that of the antenna.

 

When tuning an antenna you will see a nice SWR dip where the antenna is resonant because the impedance of antenna drops at the point it is resonant.

 

The miraculous NanoVNA can be your friend because it affords you the ability to calibrate to the point of antenna connection, then focus on analyzing the antenna itself to see what it’s actual doing, seeing what its real impedance is. Other analyzers can be just as useful. In using one of these at the antenna you can see the effect of every change you make, from moving it, mounting, nearby surfaces, lengthening and shortening.

 

All in all, 1.6:1 to 1.7:1 are not bad at all. So, except for the learning you would achieve, I see no critical reason to fret about the values you have listed.

 

So building out my system for the truck. Finally, after a year of laziness I am building a center console (well the front half) to house a FTM400xdr and an Anytone at6666 10/12m radio (code for illegal cb radio for those that don't know). It will be used for 10m work as well.

 

STAY TUNED FOR A BUILDOUT POST.

 

So the antenna is mounted to the hood in front of the a pillar. Using a NMO34 with a 64" whip (10m) and a NMO30 with 64" whip(11m).This is going away. The whip is .100 and it flexes to 45 degrees above 30 mph and much more at 70. Incoming MFJ 10m hamstick. Should be tunable for both bands.

 

I was trying to hold to Marc's talk about every commercial antenna for 11m is too short even with a spring. SIDE NOTE: I DID FIND SOME ONE THAT MAKE CUSTOM EXTENDERS TO GET A 102" TO 115" OR SO.

 

Now back on track. The nmo34/30 combo the lowest I could get was a SWR of 1:1.6 or 1:1.7 with trimming. I gave up because the SWR started climbing back up. Lowest frequency for the 10m set up was 28.594 (VSWR OF 1:1.6). Just a hair away from the target of 28.400.

 

It started making me think of antenna resonance and how to calculate it.

 

So in order to find the resonant frequency of an antenna I am looking for an impedance of about 75 ohms purely resistive? This the value I gather for a dipole in free space.

 

So the question is how the hell do I find resonance of a custom, less then ¹/4 wave whip with a bottom loaded coil?

 

 

 

Sent from my SM-G975U using Tapatalk

 

Michael

WRHS965

KE8PLM

[kidphc]

I have seen the video before and it was no help.

 

Michael thanks for the input. I guess this antenna coil combination wants to stay resonant at 28.59 or so. Uncut it seamed to be resonant at 29 mhz, with the swr at 1:1.7. Currently, it's 2.0 swr at the band edges and at 1:1.7swr at 28.450, 1:1.66 at 2.58.

 

I guess next step is to check the impedence values at those frequencies.

 

When the nmo30 and hamstick comes in about a week I will have to rerun the sweeps.

 

The 1:1.7 swr I started to think was due to the terrible ground plane interactions. Bending the antenna away from a pillar resulted in an swr that dropped a few hundreths.

 

I will never recommend a hood mount that close to the pillar as an even remotely acceptable mounting location. May have to get another mount that mounts the antenna closer to the center of the fender. Should be intresting to compare the two locations and their respective swr values.

 

 

 

Sent from my SM-G975U using Tapatalk

[SUPERG900]

Hmm... my NanoVNA died after about two weeks of use. The build quality on my particular example left something to be desired.

 

I've replaced it with a Mini1300, which is larger and somewhat easier to use. It'll work as a simple SWR meter, swept SWR, VNA, and will display resistive and reactive components. Very nice.

 

Since the micro flash card it came with was dead, I had to buy a replacement card, and then perform a HW cal, which required opening the unit up and soldering a jumper. Other than that, it's worked fine. More expensive than a NanoVNA, cheaper than a rigexpert.

[WyoJoe]

I have a nanovna ver h4.

 

Kinda why I am asking how you would look for the resonance. It certainly has way more ability then the user to do many functions at this point.

You can find instructional videos for the NanoVNA on Youtube. Everything I've learned to do with mine was learned from Youtube.

[mbrun]

I remember back when I obtained my NanoVNA and was doing experiments with a simple 1/4 wave ground plane. It allowed me to not only see the resonance change as I trimmed the antenna down to size, but it also allowed me to observe the impact of length and descending angle of the ground plane radials had on the impedance of the antenna. Without it those factors are hidden mysteries.

 

I have seen the video before and it was no help.

 

Michael thanks for the input...

 

The 1:1.7 swr I started to think was due to the terrible ground plane interactions. Bending the antenna away from a pillar resulted in an swr that dropped a few hundreths...

 

Sent from my SM-G975U using Tapatalk

 

Michael

WRHS965

KE8PLM

[kidphc]

I remember back when I obtained my NanoVNA and was doing experiments with a simple 1/4 wave ground plane. It allowed me to not only see the resonance change as I trimmed the antenna down to size, but it also allowed me to observe the impact of length and descending angle of the ground plane radials had on the impedance of the antenna. Without it those factors are hidden mysteries.

 

 

 

Michael

WRHS965

KE8PLM

That kinda was the question to begin with. How to see resonance frequency not using SWR.

[tweiss3]

I would suggest modeling the antenna in 4NEC2, but you would have to know the information about the base coil, like capacitance. It would get you an idea of the resonance and the radiation pattern. I can't remember if it will tell you the SWR or not. It has been a while since I dabbled in that.

 

What about buying a screwdriver antenna, or something like the Yaesu ATAS-120A?

[kidphc]

Its a Larsen NMO34 coil. Unfortunately, no information on capacitance. Might have to go old school and break out a mult-meter to measure it out. Thanks for the information it is a start.

[tweiss3]

I just looked back, I used MMANA-GAL Free. I was wondering how my patter changed on my EFHW with sag as it was actually installed in the field: https://1drv.ms/b/s!AmNCKj05x9hYgf5PCTQOZ_HD_zUvkg?e=jcTW11

 

I did not put in information for the matching transformer, so the SWR did not calculate, but there are places to put that in.

[mbrun]

That kinda was the question to begin with. How to see resonance frequency not using SWR.

Here is a snap shot of my NanoVNA showing the resonance of the external GMRS at my home. The resonance can recognized by the sharp dip in the yellow LogMag trace. Since the impedance of the antenna (as depicted in the green smith chart) is also closely matched to the 50-ohm coax there is also a corresponding dip in the SWR. So this image shows that, in my case, the best SWR reading corresponds with the resonance of antenna.

 

So, in part, how can you view the resonance of an antenna independent of SWR? Use a NanoVNA, perform an S11 measurement with the instrument set to display the LogMag trace.

 

Hope this helps.

 

.

 

 

 

 

Michael

WRHS965

KE8PLM

[kidphc]

Here is a snap shot of my NanoVNA showing the resonance of the external GMRS at my home. The resonance can recognized by the sharp dip in the yellow LogMag trace. Since the impedance of the antenna (as depicted in the green smith chart) is also closely matched to the 50-ohm coax there is also a corresponding dip in the SWR. So this image shows that, in my case, the best SWR reading corresponds with the resonance of antenna.

 

So, in part, how can you view the resonance of an antenna independent of SWR? Use a NanoVNA, perform an S11 measurement with the instrument set to display the LogMag trace.

 

Hope this helps.

 

.

 

 

 

 

Michael

WRHS965

KE8PLM

Damn it. That was what I was looking for. I am looking for the lowest log?

 

Sent from my SM-G975U using Tapatalk

[tweiss3]

Damn it. That was what I was looking for. I am looking for the lowest log?

 

Sent from my SM-G975U using Tapatalk

 

Looking for the mix of both the lowest Smith (impedance) and SWR, but you can always improve the match if you get good swr by adding a capacitor to bridge the antenna and ground (shield).

[WRNA236]

Don't get too wrapped around the axle watching SWR.  It's one aspect of tuning an antenna.  It's important but having an absolutely perfect 1:1 SWR is no guarantee of a good antenna.  A purely resistive dummy load will show 1:1 VSWR but radiates essentially zero RF energy.   An incandescent light bulb will radiate more RF while showing a terrible SWR than a dummy load.

For example, a 1/4λ monopole has a characteristic impedance of approximately 36Ω and thus a SWR that calculates to 1.38:1 when it's perfectly tuned for resonance with a 50Ω feed and actually exciting electrons well.  In the real world a SWR from 1.1:1 to 1.5:1 for such an antenna indicates a very good antenna when you figure in non-ideal ground, coax loss, etc. 

In the impedance plot and Smith chart you'll see the effect of everything, the connectors, the coax, the antenna and its ground.  That's the advantage of using a VNA or even just an SWR bridge or antenna tuner like the MFJ is that you can see the dip and trends.  Point being it allows you to see if your measured values jive with your expected values. 

Point here is 108" (102" plus a 6" spring) is very close to 1/4λ for 10m (it is on 11m) so that you got 1.7:1 is actually about what you would expect.  It'll never be 1:1 and if you get that it means you've added something that's absorbing energy to make it so, e.g. the capacitance that's been suggested. 

That's essentially building a matching network that's unnecessary in this case.  If the antenna itself was not resonant or had a very high impedance (such as a 1/2λ end-fed, which has a VSWR of around 2kΩ at resonance) you have to impedance match to your radio.  That's just what you have to do.  But the matching network doesn't make the antenna any better, it just makes it so the 50Ω output of the radio doesn't see a big reflection that will cause damage or kick in protection circuits.

But this is not necessary here.  Your radio will have no trouble with a 1.7:1 SWR.  It won't start to roll back power until about 3:1 in most cases and at 1.7:1 this antenna will be radiating all the energy it's capable of.  Like I say, your measurement validates what you expected.  Any matching will just be consuming energy and not helping make the antenna any better necessarily.

 

 

[kidphc]

Funny enough swapping to a 10m hamstick netted 1:1.12. Well till it fell off on the highway and got run over.. now it is a bit higher. Lol

Sent from my SM-G975U using Tapatalk

[WRNA236]

7 minutes ago, kidphc said:

Funny enough swapping to a 10m hamstick netted 1:1.12. Well till it fell off on the highway and got run over.. now it is a bit higher. Lol

Sent from my SM-G975U using Tapatalk
 

Hamsticks can show a closer to 1:1 due to their nature as shortened physically but 1/4λ electrically.  So there's some coil loss.  They have the same ground limitations as a full length antenna, though.

BTW, remember SWR is logarithmic.  So 1:1 means no reflection but 1.12:1 is only 0.32% reflected.  IOW 100 watts in and 0.32 watts was reflected.  At 1.5:1 you see 4% reflected, 2:1 reflection is about 10% and 3:1 about 25%.