I recently returned from Afghanistan, where I encountered a number of nifty radio toys. One of the neatest toys I saw was the Stealth Technologies ST-940B HF Mobile Magnetic Loop Antenna:
After reviewing the specs on the antenna (and receiving a price quote of $5000 US from a distributor!) I became very interested in building an HF mobile magnetic loop of my own (and hopefully for much less $US). In addition to its compact size, the magnetic loop antenna offers several distinct advantages over a traditional HF mobile vertical antenna, which makes it a very attractive option for those desiring enhanced mobile HF performance. This article is the introductory article of a series I plan to post as I document the construction of an HF mobile loop antenna for my Jeep.
Traditional HF mobile vertical antennas commonly use a whip and loading coil configuration. Losses in mobile HF setups can be enormous. The ARRL Antenna Handbook categorizes general HF vertical efficiencies at around 80% on 10m; however, efficiency can drop to around 0.2% on 80m. This means that a 100W radio will only radiate .2 watts on 80m using a traditional HF vertical. Even an incredibly efficient HF vertical with cap hats and a large diameter, high-quality loading coil will only achieve around 2% efficiency on 80m. The two major losses in an HF mobile vertical are coil losses and ground losses. Magnetic loop antennas mitigate both of these to provide increased performance.
For those unfamiliar, a magnetic loop antenna is almost as simple as it sounds. A loop with a circumference of 1/8 to no more than 1/4 wavelength is formed out of copper or aluminum, fed on one side, and matched on the opposite side. The two main feed types are an inductive loop formed at the feed point, or a shunt feed using a gamma match. The loop can be matched with something as simple as a coax stub, resulting in a fixed frequency loop. More advanced loop designs use a variable capacitor to achieve a match on a range of frequencies. The loop does not necessarily have to be circular. Loops can also be octagonal, square, or rectangular. The more area encompassed by the loop, the better the performance, thus the most efficient loops are circular; however, square or rectangular loops exhibit minimal performance differences and can be more practical for some purposes (like a mobile HF loop installation.)
Coil losses in an HF vertical antenna are a function of the coil Q-value. The higher the Q-value, the lower the losses in the coil; however, even the highest quality coils used in modern verticals are only capable of a Q of around 300. In contrast, a magnetic loop is often capable of Q-values well above 1000. An 18 foot circumference square magnetic loop made of 1" copper pipe will have a Q-value of around 1370 on 80m. The high Q-values of the magentic loop contribute to its superior performance in the mobile environment.
The greatest loss that most HF verticals experience is ground loss. Again, magnetic loops perform very well in this environment. A vertically mounted magnetic loop sees very little ground loss, even when mounted very close to the ground, resulting in significantly better performance when compared to an HF vertical antenna. It is important to note that this applies only to vertical loops, as horizontal loops should be mounted at heights comparable to dipoles for a given band to achieve best performance. Another significant performance characteristic of vertically mounted loops is a strong radiation pattern from 60-90 degrees elevation. This characteristic makes vertical loops very well suited for NVIS operations.
Again, for the unfamiliar, NVIS is an operating strategy which relies on high angle radiation below the MUF to generate signals reflected off the ionosphere which provide very uniform radio coverage for a 300-500 mile radius around the transmitting station. This type of operating eliminates the "skip zones" which are present with lower-angle radiation, and makes for very effective regional communications for emergency services, as well as in terrain which prohibits traditional line-of-sight (LOS) operations on VHF/UHF frequencies. In the amateur bands, most NVIS communications utilize the 40m band during daylight hours, and the 80m band after sunset; however, the anticipated changes in FCC regulations with respect to the 60m band have also made it more attractive to me for NVIS operations due to its good performance during nighttime operations, coupled with a dimensionally smaller antenna requirement. I wanted NVIS operation capability in my mobile HF setup, as I anticipate it being used primarily for contingency communications in my local/regional area, or for rugged terrain trail runs where traditional VHF/UHF comms are not suitable.
In building my loop antenna, much of my pre-build planning was accomplished using the KI6GD loop calculator:
This tool helped me quickly assess the performance factors I would be considering in the construction of my loop antenna. Although the KI6GD calculator does not model ground-plane "half-loops" such as the ST-940B (where the cargo rack is an integral part of the loop), I anticipated that conventional loop modeling would give a rough idea of expected "worst-case" performance, and actual performance would be slightly better than computed. The KI6GD software allows the user to quickly change loop circumference, conductor diameter, operating frequency and operating power to view calculated loop performance.
I began my calculations focusing on the 80m, 60m, and 40m bands due to my interest in NVIS work with the mobile antenna. I planned on building the roof-rack antenna out of 1" copper pipe over a copper mesh floor, with square steel tube as the structure material. I initially planned a roof rack measuring 40"W x 60"L to fit the YJ, knowing I could increase the size if needed. This would yield a vertical loop 5 feet long and 20" high (assuming the loop bar is designed to fold flat along the edge of the roof rack.) Plugging in the relevant numbers into the KI6GD calculator yielded the following output for 80m (calculations are for a square loop; however, rectangular loop numbers should look very similar):
From this output, we see that the loop's efficiency on 80m is calculated to be 3.5%, which rivals, if not slightly exceeds the performance of a high quality HF vertical antenna. Keep in mind the high angle of radiation, which makes this a much better NVIS antenna than a traditional HF vertical. Also keep in mind the dimensions of the loop are 60"L x 20"H...a very compact package for 80m.
Although the antenna promises reasonable performance on 80m, as mentioned above, the new regulations which up the ERP on 60m to 100W and allow both CW and digital mode operation piqued my interest and provide great promise for NVIS operations. The 60m band is suitable for NVIS operations, and offers greater efficiency for night NVIS as evidenced in the following calculation (60m Channel 1 was used for this run):
The efficiency at 60m jumps up to almost 10%...a significant increase over performance on 80m. Stepping up to 40m shows even better results:
The 15m band is the highest this antenna will tune. Above that, the loop is too large to function as a small transmitting loop. At 15m, however, the loop's performance is excellent:
It should be noted that although the loop radiates well at high angles, it's low angle performance is also acceptable and should yield some DX work on the higher bands (20m, 17m, and 15m). The following NEC antenna diagram and radiation pattern show the general coverage provided by the antenna:
[Figures provided by www.smeter.net]
In addition to loss reductions associated with the magnetic loop, the antenna is also less susceptible to capacitive noise (power line noise, for example) which creates a higher SNR and subsequently improved performance. The high Q of the antenna creates a very narrow bandwidth, which can also aid in filtering out adjacent noise to a given signal, thereby also enhancing SNR.
If we increase the length of the loop bar (and roof rack) from 60" to 72", the performance of the NVIS bands is noticeably improved. Efficiency on 80m jumps from 3.5% to 5.2%. On 60m, efficiency jumps from 9.7% to 14.1%. On 40m the jump is from 23.6% to 32% efficiency. Increasing the loop conductor size to 2" provides an even higher jump...80m efficiencies approach 10% while 40m efficiencies approach 50%.
The antenna can be matched across the band from 3.9 to 21.350 MHz at 100W with a variable capacitor capable of 10-500pf and a 5kV rating. An air- or vacuum-variable capacitor can be driven by a simple 12V stepper motor using a control switch mounted inside the vehicle. With this configuration, tuning the loop is as simple as setting the desired operating frequency and adjusting the stepper motor until received audio noise is the loudest. Fine-tuning can be accomplished with the TUNE function on many radios and an inline SWR meter.
Overall, the mobile loop antenna provides excellent performance in a very compact package. In coming posts, I will document the construction and installation of the loop, as well as its performance once installed. Until then...