# Receive only antenna for 160m – matching and performance discussion – 8000pF?

A reader of Receive only antenna for 160m – matching and performance discussion referred me to an online discussion with a simpler solution.

A couple of quotes from posters…

It almost doesn’t matter what the actual receiver Zin is. What matters is the 100ft of 50 or 75 Ohm coax that is used to get the Rx-only antenna signal from where the antenna is placed to the receiver.

and…

That’s what I thought, also.

A few hundred feet of coax can be ~8000 pF of parallel capacitance which should short out signals by having only about 11 Ohms of reactance.

So the gist of this is that:

• receiver Zin does not matter;
• the 100′ of coax or 300′ of coax matters; and
• 300′ of coax can be adequately represented by a shunt capacitance of 8000pF.

Receiver input impedance is not necessarily a tightly controlled parameter, but it is measurable, so lets take a real example.

## IC7300 example

Let’s measure Zin of an example IC7300 around the 160m band.

Above is Zin plotted from a saved .s1p file. At 1.85MHz, Zin=56.2-j5.4Ω, a little off nominal, but pretty close (VSWR50=1.35). The dotted line is X and the dashed line is R, ignore the solid line… it is for people who don’t understand impedance.

## 8000pF approximation

Above is a model that takes the measured receiver Zin and shunts it with 8000pF, and calculates the load seen at the antenna terminals. That load is 1.9-j10Ω which would introduce a large MismatchLoss (15dB) to the Pennant example discussed.

## But, is the approximation valid?

Well, the length of coax is a transmission line, and whilst under some circumstances it might be reasonably approximated as a shunt capacitance, we really only know by calculating the transmission line solution.

Above is a model that includes 100m of RG6 transmission line. I must note I have reservations about the way transmission lines are modelled in SimNEC, but for the purpose of this discussion, the model is adequate.

Above is a model that takes the measured receiver Zin and cascades it with 100m of RG6 transmission line, and calculates the load seen at the antenna terminals. That load is 64.8-j12.6Ω which would introduce a modest MismatchLoss to the Pennant example discussed, but not nearly as bad as pretending that 8000pF is a sufficiently good approximation of the coax.

Above is a calculation of the MismatchLoss, 1.84dB, quite modest

Above is zoomed in on the graph. Sure, impedance varies with frequency, partly due to the receiver, partly due to standing waves on the 75Ω line, but the excursions are not huge.

It is quite feasible to make a 4:1 broadband transformer that is close to ideal, so we can get a good estimate of the behavior of such a transformer by specifying Zo as the conjugate (because of the way SimNEC works) of the Pennant source impedance divided by 4.

Above is the amended model. The MismatchLoss is now 0.1259dB, total loss is 1.635dB, and loss in the 100m of RG6 under the standing wave conditions is 1.509dB. A practical transformer might add loss of tenths of a dB.

## Conclusions

• The theory and discussion of the previous article stands, the referenced online discussion on social media is quite misguided.
• The FUD (Fear, Uncertainty and Doubt) are unwarranted, the scenario calculated here has quite modest MismatchLoss. In fact, the entire system has quite low loss.
• Read widely, think carefully, be guarded about blind acceptance of social media discussions.