VALIANT II Intermittent -- QUIZ & ANSWERS

[AB2EZ]

Bear

It is interesting that either of the following will significantly increase the RF load impedance (at the operating frequency) that is seen by the RF output tubes... looking into the pi network. Therefore, causing the average plate current, at resonance, to drop significantly... due to the drop in the instantaneous plate voltage on instantaneous plate current peaks:

a. A significant reduction in the RF load resistance that the output RF connector is looking into (e.g. an RF short circuit)

b. A significant increase in the RF load resistance that the output RF connector is looking into (e.g. the antenna system becomes disconnected, due to a bad RF relay or a bad RF connector)

The RF load resistance (at the operating frequency)... that is seen by the tubes looking into the pi network... is equal to:

Q² x R_eff = [2 x pi x f x L]² / R_eff,

where L is the inductance of the pi network's inductor, and R_eff is the effective RF load resistance across the output of the pi network. Note that it is inversely proportional to R_eff

R_eff is always less than R_ant... the RF resistance of the antenna system that is connected to the output RF connector.

R_eff = R_ant/[1+ (2pi x f x R_ant x C_loading)²]

For a given amount of loading capacitance, the maximum value of R_eff occurs when:

2pi x f x R_ant x C_loading= 1

Separately, I believe that Dow used the spring loaded connector design, with the internal gap that opens and closes (intended to be on the receive side of the relay), to provide extra isolation of the receiver when the relay was in the transmit position.

Also note: I have found that the SO-239 receptacles on DowKey relays, and also SO-239 chassis receptacles, tend to make intermittent contact with the center pin of a PL-259 plug... particularly if a PL-259 plug with excess solder on the outside of the center pin has been previously inserted into the SO-239. To fix this problem, I have resorted to inserting a 1/4 inch pin... made of rolled up aluminum foil... into the SO-239 socket... to improve the contact.

Stu

[w3kmp]

Nice post, with great pic's. I liked giving the clues, and seeing who can come up with explanation.

[WBear2GCR]

Thanks!

I thought that doing it this way - and assuming I was actually able to find and fix the problem - would be more interesting and fun than just posting up a repair. I have to admit that I had an underhanded motivation too, that was to keep me from wandering off the repair. Once I went "public" I had to save myself the embarrassment of not getting it done or not getting it fixed.

Stu, thanks for the theory. I assume that R_eff means "effective"?? Not "efficiency".

So, what is the relationship between Q and R_eff? Looks to me like those two equations can be substituted, and it also looks like the L value replaces the C if it is done that way... them dar matty maticks be like cipherin'!

                   _-_-

[AB2EZ]

Bear

Q= L/R_eff, where L is the inductance of the pi network's physical inductor... adjusted to take into account the reactance associated with the series equivalent circuit of the loading capacitor in parallel with the antenna system.

For example, if j x 2 x pi x f_resonant x C_loading = j/(50 ohms)= j x 0.02 siemens,  and

if the impedance of the antenna system is 50 ohms...

then the equivalent series circuit for this parallel combination is

[50 ohms ] x [ 1 - j] /2 = 25 ohms - j25 ohms

Therefore, in the equation for Q,  L = L_{physical inductor} - L₂, where:

L₂ = 25 ohms/(2 x pi x f_resonant)

The impedance, at the resonant frequency, looking into the pi network is:

Q² x R_eff

For example, if the loading capacitance is set to minimum (~0pF), then Reff is the resistive component of the impedance of the antenna system.

If R_eff = 50 ohms, and Q=10, then the impedance looking into the pi network at the resonant frequency is 10 x 10 x 50 ohms = 5000 ohms.

Also, since Q= L/R_eff, it follows that:

Q² x R_eff = L²/R_eff.

All of the above can be derived by applying simple circuit theory (e.g. converting the parallel combination of a loading capacitor and a 50 ohm antenna input impedance into an equivalent series circuit), and algebra.

Stu