Field report from 5B4AIY on the I-PRO Traveller portable antenna

Resonance tricks: inductance and capacitance for compact antennas

In order to obtain resonance, loading coils are introduced, note however that they do not make up for the missing length, instead their inductance is designed to cancel the inherent capacitive reactance of a shortened antenna leaving only the resistive part of the input impedance to be dealt with.

Unfortunately loading coils themselves have their disadvantages. They are lossy because the RF resistance of the coil is greater than its DC resistance as a result of skin effect where RF currents only flow along the outer surface of the wire, and do not completely penetrate the conductor.

In addition, they have distributed capacitance of their own, and this both counteracts their inductive effect as well as causing the inductor to self-resonate at some frequency.

To reduce these effects it is common practice to add capacitance to the antenna in the form of ‘capacity hats’ at the ends of the dipole. This both reduces the amount of inductance required to achieve resonance, as well as moving the high current radiating portion of the antenna further away from the centre which improves performance. In this antenna these ‘capacity hats’ are the fold out tubes at the ends of the top and bottom sections of the antenna.

Because the resonant frequency of the antenna is somewhat dependant upon its location, the conductivity of the ground, and the presence of other nearby metallic objects, the lower capacity hat fold out tubes are used to alter the end capacitance of the antenna and thus achieve resonance on the selected band. By folding the arms upwards, the capacitance is reduced, increasing the resonant frequency. Conversely, folding the arms out and down the capacitance is increased, lowering the resonant frequency.

Setting up and tuning a versatile antenna

The centre section of the antenna consists of a number of loading coils electrically connected in series, with jack sockets used to select the required band configuration. In order to connect to the transceiver there is an internal balun allowing the connection of a coaxial cable by means of a PL-259 plug to the short SO-239 terminated connecting lead.

To prevent pickup along the length of the coaxial cable it is important to ensure that it droops centre section to which the coaxial cable is clipped.

Setup is straightforward. The base of the antenna is unfolded and set down on the ground, and with the aid of the supplied spirit level the angles of the four feet are adjusted so that it is vertical. The spirit level is removed and stored in one of the feet, and then the centre section has the top capacity hat attached with the arms folded out. The bottom section is attached and slid into the base section, and the centre section complete with the upper capacity hat inserted. The connecting cable is plugged into the jack sockets appropriate for the band in use, and the lower capacity hat’s arms folded to the angle giving the lowest SWR. This all sounds more complex that it is, but after a few repetitions, it becomes second-nature.

If you are using an Icom IC-7300, or IC-705, then there is a built-in SWR sweep setting in these transceivers that can be used to obtain the best match. Otherwise, using an in-line SWR bridge and a few watts from the transceiver the best match can be fairly easily found. It is worthwhile spending some time making these adjustments for each band and recording the fold-out angle so that it can be easily re-selected.

Scientific analysis and global reach

So much for the construction and setup, but how well does it work? Most users will comment at this point on the stations worked, but this does not take into account propagation, and band conditions.

I prefer a more scientific approach using a WSPRLite transmitter over a 24-hour period to record where my signals were received and their relative strengths. This transmitter has an output power of just 200mW, but nonetheless can be received all over the world when conditions are reasonable.

The first test was to compare the I-Pro with a known antenna, in this case my Hexbeam. This entails using two synchronised WSPRLite transmitters. Unfortunately I could only conduct this test on 20m, as my WSPRLite ‘Classic’ transmitter’s highest frequency is 20m, whereas my other transmitter, a WSPRLite ‘Flexi’, can cover all the way up to 6m. Nevertheless this test was conducted over a 24 hour period with the following results.

The maximum possible distance is 20,000km, but this would be to the antipodal point with respect to Cyprus. This location is in the middle of the Pacific Ocean, and there are no land masses in the vicinity. Therefore, this could never be obtained, but note that the ‘reach’ of the I-Pro Traveller is only slightly less than the Hexbeam.

Impressive results in the 24-hour performance test

The next series of tests show the performance of the I-Pro antenna on the 20/17/15/12/10m bands. In all cases the tests were run for a period of 24 hours. The antenna was left up on the bank in the field behind my house with a clear view of the horizon all around, and no obstructions. My QTH is approximately 587m / 1,925' above mean sea level.

Optimization and practical check

• The knobs securing the base fold out supports required a large amount of torque to tighten them to secure the legs. I found that by replacing the small washers with larger 25mm washers this significantly increased the clamping area and reduced the amount of force required.
• During the week long test there was a rainstorm, as well as a large drop in temperature such that the resonance points of the antenna were significantly altered. Having now deployed the antenna in several different locations, an antenna analyser is almost essential to ensure optimum setup.
• Setup and tear-down is by no means a quick operation. In my case I found that one should allow about 30 minutes for both setup and tear-down.
• Setup and tear-down in anything other than still-air conditions can be a challenge, bearing in mind the 3m height of the assembly. Equally, in windy conditions the Universal Guying kit is definitely required.
• The guy rope locking pieces of the Universal Guying kit loosened after a short time, and required constant attention. There needs to be a rethink about these friction pieces as they are not particularly secure.
• I found the spring loaded buttons that latch the top and bottom sections in place were difficult to operate, especially in the cold. Using my lathe I made a small tool with a 3mm projection with a machined recess that greatly aided in pressing the buttons in allowing an easier assembly and removal.

Conclusion: compact antenna, powerful performance

Overall this antenna performed well, especially considering its small size in comparison to the wavelengths being radiated.

In addition to the image data that can be accessed from the DXExplorer website, there is also statistical data in the form of a table for each test that can be used for more detailed analysis. Adrian Ryan, 5B4AIY, will be happy to provide this.

Adrian Ryan, 5B4AIY 07/December/2024