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After the subject "Messure The Characteristic Impedance of A Line" for which I thank you for your precious and assiduous participation, offers you technical curiosity.

I made two antennas combining 4 pairs of coxial traps without worrying too much about their operation, the objective being the proper functioning of the antennas.

I use the trap.exe whose origin I can no longer find but donation I show you the screen in pj.

For the test, it is a hatch which was intended to operate on 21 MHz but which turned out to be badly calibrated. This hatch rose in shunt on port 0 of a nanovna which allows the S11 to be measured. Practically it resonates on 19,377,700 MHz (measured). We see in photography how the coaxial hatch is wrapped. Coarsely the soul and the braid are sores in series and the soul/braid capacity makes the condenser of agreement. Finally, that's what we believe.

The software tells us that the length of coaxial used represents 74.17 PF. In the calculations made under Excel, I supped in the ohm reagent curve brought back by the trap, the reactive curve that a classic LC plug would bring back.

In the example, I imposed that the points A and B are combined. Just solve the system with 2 unknown equations. The corresponding cap appears composed of a capacitor of 20.27 pf.

I had noticed by building my antennas that the resonance frequency of the traps did not seem critical. This calculation would give an explanation: the Q in charge, because of C small, remains weak.

I therefore submit this curiosity to your sagacity, knowing that I could be wrong in my measurements or propose an exotic comparison.

I asked a question about [email protected] asking how you could simultaneously a coaxial trap. I had no answer. Do you think that my modalization towards this type of LC plug would be suitable.

Good WE, 73
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F1AMM
Fran?ois
(sorry for my English; I am a French old OM)

The coax trap software is by VE6YP .

There are two ways of connecting the coaxial ends:
Low Z: Shield and Center on one end, only the center on the other end. The software shows the L, C and resonant frequency for this connection.
High Z: Shield on one end and center from the other are shorted. The unused center and shield provide the trap connections. This is what you have. For this, the L should be 4 times what is shown by the software. High Z L = 4 * low z L. Correspondingly, the High Z C = Low Z C / 4. The resonance frequency is unchanged.

Hope this helps.

Raj, N2RD

Fran?ois, this shows how to optimize an antenna with traps. It uses a trapped tribander as an example. What might interest you is that optimum trap resonances were way outside the expected bands. I was unaware of this trick, but it has been known for a long time.



Brian

Better than that, shows two ways to design a trap for a dual-band antenna:

1) Trap resonant at upper frequency; therefore adds inductance to lower band. The higher frequency element is its 'normal' length, lower frequency element is shorter than 'normal' (no inductive reactance) length.

2) Design trap to be a) inductive at lower frequency; b) capacative at higher frequency.? Then the lengths of the higher-frequency element interact with the lower-frequency element, and you shouldn't use (nor should you find) the 'normal' resonance lengths for the bands are used in the rest of the antenna.

Way To Tell:? if adjusting next-lower element makes the higher frequency lengths 'wrong', then you have the Type 2 above

Triband traps and beam adjustments are left as? an exercise to the reader ('Jackson problems?!').

WA0VYU

--
Best Regards,
Roger WA0VYU

Hello

High Z L = 4 * low z L. Correspondingly, the High Z C = Low Z C / 4. The resonance frequency is unchanged.

** Could you elaborate on this passage a bit more. Where did you find this explanation or did you figure it out yourself? I have difficulties to understand

In my case example
The computer indicates 19.377 MHz 74.17 pF
The realized hatch resonates on 19.39 MHz so ok.

I measure the S11 in shunt and I calculate Z=A+jB A remains weak; there is only the reagent jB

I'm calculating an LC cap which is going to have roughly the same jB curve.

The math tells me an LC with a C of about 20 pF. That is therefore a ratio of approximately 4 as you indicate. So your assertion seems to be true.

1) Trap resonant at upper frequency; therefore adds inductance to lower band. The higher frequency element is its 'normal' length,
lower frequency element is shorter than 'normal' (no inductive reactance) length.

** So far I have used these coaxial traps in the case of your type 1.
--
Fran?ois

Owen Duffy, the author of this article just sent me that link. It is a good and comprehensive explanation of what is going on. It shows that the high Z estimate of 4 times low Z is an approximation of the actual impedance that is affected by the transmission line nature of the coil.

Some more experiments to try:
1. Measure the inductance at a lower frequency, say three octaves down, i.e., for a 7Mhz trap, measure at 1MHz. At this frequency, the impedance of the capacitance is much higher and helps dealing with stray capacitances.
2. Measure the capacitance at a higher frequency, say 30Mhz for a 7Mhz trap. At this frequency, the impedance of the capacitance is much higher and helps dealing with stray capacitances.

Best,
Raj, N2RD
--
Rajiv Dewan
rmdewan@...

Owen Duffy, the author of this article

just sent me that link. It is a good and comprehensive explanation of

what is going on. It

It's very good because this article is in HTML and Chrome translates it to
me in French.

I wonder how VE6YP software . was
established because it is remarkably accurate.

Concerning the question transmission line or not, here is an example which
surprised me a lot.

I made a double doublet that you can see in this picture.

- The top doublet was cut for 21.241 MHz, I added a 21.241 trap to it and
added a length to cover the 18 MHz band
- The bottom doublet was trimmed for 28.850 MHz, I added a 28.850 trap to
it and added a length to cover the 24 MHz band

As you can see in the picture, the length of wire behind the trap is
ridiculously small, as if the trap lengthens the antenna more than adding
choke to it.

I need to make a new 40m trap. I will take the steps you suggest. I have a
LF bridge (100Hz - 1khz - 10kHz - 100kHz) which will be fine for
cross-checking at 100kHz with the nanoVNA.

73
--
F1AMM
Fran?ois

-----Message d'origine-----

De la part de Raj Dewan, N2RD

Envoyé?: dimanche 28 mai 2023 17:20

the url of the image has disappeared


The rest is here

--
F1AMM
Fran?ois

The impedance of the trap was measured with the S11 shunt. Fo is the (measured) resonance frequency of the trap.

In the attachment [C equivalent.png] we see the value of the cap capacitor equivalent to the trap calculated in such a way that Z trap = Z cap and that the resonance frequency of the cap is the same as that of the rap.

The objective is to be able to simulate in EZNEC a coaxial trap by an LC circuit.

On the graph we can therefore read that if we use a resonant trap on 19,414,769 MHz (measured), if we want a relevant simulation at half frequency (9,707,384 MHz) we must simulate the trap with a 29 pF plug / / 2.32?H. This simulation will *only be correct* for Fo and Fo/2

Note that the value of the capacitor formed by the coaxial is 72.5 pF

I have attached the corresponding Excel workbook to allow you to check the calculations if you wish. Also Trappe.sp1

Bridge measurements
100 Hz R=0.04 ? L=3.9 ?H
1 kHz R=0.04 ? L=3.89 ?H
10 kHz R=0.05 ? L=3.78 ?H
100 kHz R=0.1 ? L=3.61 ?H

As desired, attached, the evolution of the parallel capacity presented by the trap between 30 and 50 MHz [C parallele trap.png]. Nanovna-saver only knows how to display the value of the series capacitor, which is of no interest here.
--
F1AMM
Fran?ois

Sorry, my previous message is almost ALL wrong.

The attached graph is much more reassuring for integration into EZNEC. The values of L and C are almost constant with the frequency.

The 20 pF remain to be compared to the 72.5 pF of the coaxial trap Design software (ratio 3.6, almost 4). We can therefore say that a coaxial trap behaves like a parallel LC circuit.
--
Fran?ois