Background
1. There have been many questions concerning the variometer used with the 19 Set. In fact, it is nothing more than a variable inductor used to tune the 12 foot whip to "look like" an electrical 1/4 wavelength.
What is not understood is how the low radiation resistance of the 12 foot whip is matched to the output of the transmitter. Earlier research and test bench measurements indicates that the output impedence of the set is approximately 40 ohms.
Therefore it would seem that a significant mismatch would result even though the variometer were adjusted to make the 12 foot whip resonate.
2. Recently, members of the "19 Set Group" have been using the sets for communication over distances in excess of 500 Km.
The "3/4 Wavelength Inverted L" antenna (actually 1/2 wavelength dipole, end-fed through the variometer and 12 feet of insulated wire), has been particuilarly effective in this role.
It was suggested that by the addition of a 7 MHz trap, a "Dual-Band, 3/4 Wavelength, Inverted L" could be constructed which would be much more convient to use than a similar antennae using insulated links and shorting clips.
3. The design of such an antenna is problematic as the loading effect of the trap is not easily calcualted. Further, as the antenna is essentially an end-fed, half-wave dipole, trimming it to resonance cannot be done in the normal manner.
Aim
4. The purpose of this paper is to document recent tests done with the 19 Set antenna.
PART II - DISCUSSION
Method
5. One proposal was made on how to tune the trapped antenna. The suggestion was to carefully note the variometer settings when the set was used with a 12 foot whip.
Then, with the "Dual-Band, 3/4 Wavelength, Inverted L" attached, and the variometer setting unchanged, trim the antenna for maximum antenna current. This would occur on 7 MHz when there was exactly 1/2 wavelength of additional wire added between the high end of the 12 foot section and the start of the 7 MHz trap.
A similar procedure would be used on 3.7 MHz with adjustments being made by adding wire to the far-side of the 7 MHz trap.
6. With the 12 foot whip, confirmation that the variometer had achieved resonance was to be gained by measurements made at the input to the variometer (ie looking at the antenna through the variometer).
A Grid Dip Meter, SWR Analyzer and Noise Bridge were to be used for this measurement.
Test Set-up
7. A Wireless Set No. 19, Mk III (sn 77160) was set-up in a clear area. The set was on "Carriers, No. 23 (ZA 10315)" with "Aerial, Seating Plates, No. 4 (ZA 10314)".
Three "F" sections (C1, C2 & C3) were used to give an antenna length of 12 feet. The set was six inches above the ground and fastened to an "earthing pin, small" by a short length of ground braid.
"Leads, Counterpoise, No. 2, Mk I (ZA 2784)" were used directly beneath the set. The set was powered by a 12 volt storage battery.
All checks indicated that the set was operating normally.
Measurements
8. The set was tuned for R/T transmission on both 3.703 MHz and on 7.050 MHz with the following results:
3.703 7.050
Antenna Current (15 V scale) 7.5 6.0
A. P. A. Tuning 3.70 7.15
Variometer Reading 87.8 111.8
10. On 3.707 MHz additional measurements were taken to confirm resonance. An MFJ model 207 SWR Analyzer was connected directly to the input of the variometer (ie the transmitter side of "Connector, Co-Axial, No. 10, ZA 3141").
No resonance was found within the range of 1.8 to 30 MHz. A Heathkit model HD-1250 Grid Dip Meter was connected to the same point through a three turn link. No resonance was noted within the range of 3.2 MHz to 6.6 MHz.
A Palomar model R-X Noise Bridge was connected to the same point and, at a frequency of 3.703 MHz, a null was found (40 + 54j ohms; ie slightly inductive).
This null was very broad and is therefore suspect. Finally, a Heathkit model HFT-9 Antenna Tuner was connected to "Connector No. 10" and adjusted to give a 1:1 VSWR (as measured with the MFJ-207 SWR Analyzer).
Resonance was achieved with setting of "C + 0.4" (transmitter), "6" inductance and "G" (antenna). This is equivalent to a "T" network with 350 pF on the transmitter side, 9 microH shunt inductance and 600 pF on the antenna/variometer side.
PART III - CONCLUSIONS
Observations on Data Obtained
11. The measurements noted in para 8 were as expected and consistant with the operating procedures and technical description of the set.
The data noted in para 10 is not as expected. The SWR Analyzer was expected to show some indications even though a perfect match was not anticipated (ie the variometer could have brought the whip to resonance but not to 50 ohms).
The Grid Dip Meter readings are the most puzzling. If there was resonance, especially if it was with a very low radiation resistance, there should have been some indication on the meter.
This points to the possibility that the APA Tuning components are also being used to bring the antenna to resonance.
This possibility is at odds with the technical description of the set. The Noise Bridge measurements are deemed inclusive due to the unusual nature of the null observed.
Finally, the measurements taken with the antenna tuner might prove useful as a check to the noise bridge measurements, however those calculations are beyond the scope of this report.
12. Notwithstanding the unexpected results obtained during the second part of the test, it is still reasonable to try tuning the dual band antenna in the manner proposed.
Although the loading effect of the trap at 3.703 MHz is not precisely known, it is anticipated that it would have a similar effect to that observed with a centre-fed, dual-band, trapped dipole (ie something in the order of 10 to 20 feet shortening).
