# Designing a Gamma Match – Simsmith design tool and confirmation of as-built antenna – comparison with Healey

A correspondent reading Designing a Gamma Match – Simsmith design tool and confirmation of as-built antenna referred me to (Healey 1969) and questioned my method.

I have tried several times to reconcile built and tuned antennas and NEC models with Healey and failed, leading me to think of the problem and devise a good approximation that did reconcile (for me).

This article attempts to reconcile the example given at Designing a Gamma Match – Simsmith design tool and confirmation of as-built antenna, an example where the two built antennas reconcile well with the ARRL published design article and NEC model.

## Example for reconciliation

The example antennas are 4 element 144MHz Yagis built around 1970. They were originally designed with a 50Ω split dipole feed, or the option of a folded dipole with 4:1 half wave coax balun.

#### Gamma arm

Gamma dimensions:

• dipole 12mm dia;
• gamma arm 12mm dia;
• gamma arm spacing 38mm;
• gamma arm length 160mm.

From this we can calculate using RF Two Wire Transmission Line Loss Calculator Zo=218Ω and length=28°.

#### Tapping point impedance – Healey eqn 1

Impedance at the tapping point:

$$Z_2=\frac{Z_1}{\cos^2 \theta}\\$$ $$Z_2=\frac{50}{\cos^2 28}=64$$

#### Transform Z2 by the gamma transmission line – Healey eqn 2 & 3

$$Z_2’=80+\jmath 103$$

#### Calculate shunt reactance of the short circuit stub – Healey eqn 4

$$X_p=\jmath 115$$

#### Find ZΓ – Healey Fig 4

$$Z_\Gamma=\frac1{\frac1{\jmath X_p}+\frac1{R+\jmath X}}\\$$ $$Z_\Gamma=\frac1{\frac1{\jmath 115}+\frac1{80+\jmath 103}}\\$$ $$Z_\Gamma=19.6 + \jmath 61.5$$

#### Apply step up – Healey Fig 3

Since the tubes are equal size, the step up from Fig 3 is 4:1.

$$Z_{in}=78.5 + \jmath 246.1\\$$

The calculated reactance does not reconcile with the Xc=86Ω to tune the built antenna.

The reactance would be nulled by a series Xc=246.1Ω leaving Z=78.5Ω which does not reconcile with the measured Z=50Ω.

## Conclusions

This attempt to reconcile the example antennas failed.

## References

• Healey, D. Apr 1969. An examination of the gamma match In QST Apr 1969.