While I’m fairly happy with the performance that I get from my first HF antenna, an MFJ-2299 telescoping rotatable dipole, which consists of an MFJ-347 mount with a pair of 16.9′ telescoping stainless steel whips which will cover from 6m-20m, mounted on an MFJ-1921 tripod, with a DXE 25′ telescoping fiberglass mast, I wanted to have an antenna that covered the lower bands, especially 40m, as well as 80m and 30m, which the MFJ-2299 does not cover.
I live in a small neighborhood in a suburb of Denver, Colorado, and have a very small lot, so stringing up 120′ of wire isn’t really an option. It’s also a new neighborhood, so the trees are all quite small, so I don’t have anything to use to get a wire antenna up in the air, nor to camouflage a stealth antenna. After doing quite a bit of research, including reading the ARRL book on “Small Antennas for Small Spaces” it seemed that the “unsung hero” of limited space may be the Magnetic Loop Antenna (MLA), or Small Transmitting Loop.
A magnetic loop antenna for amateur radio is typically made of about 10′ of large diameter coaxial cable (utilizing the braided shield rather than the center conductor) which makes about a 1m diameter loop, connected to a tuning capacitor at the bottom. A smaller inductively-coupled loop is connected to the transceiver. The capacitor can be adjusted to change the resonant frequency of the loop. An MLA doesn’t need to be mounted very high, only 1-2m, and doesn’t require ground radials, etc. Because the primary element of the MLA is a short piece of coax, an MLA is often very portable, and can be taken down and stored in a small bag, so they are popular for portable operations like SOTA activations.
There are two main drawbacks to a magnetic loop antenna — they have a high “Q” factor causing a very narrow bandwidth, and they often only handle low transmit power, sometimes as little as 10w. The narrow bandwidth means that tuning to different frequencies on the radio usually means that you have to retune the capacitor on the loop. There are some loop antennas that will handle up to 100w. For my purposes, I planned on working digital sound card modes like ft8, which are on a set frequency range, and are “weak signal” modes, so neither of these drawbacks were a concern for me. What I was looking for, though, were two features. While many QRP operators are sitting next to the antenna when working with it, I planned to be at my station in the basement, with the MLA setup outside, so I needed to be able to tune it remotely. I also wanted to be able to leave the antenna setup so that I could play on the radio whenever I had free time, without having to spend a lot of time setting it up and taking it down again. This meant that it needed to weather-resistant.
One of the options that I considered was the MFJ-1788 MLA, which is a 36″ diameter loop that is remotely tuned and handles 150w, and can be mounted outside. It’s designed for fixed installations, though, so it’s not exactly portable. I wanted to be able to take my antenna with me when camping, traveling, or for portable use, so this wasn’t a good fit for me. I also considered options like the Chameleon Antenna F-LOOP Plus 2.0 and several options from Alpha Antenna, neither of which offered remote tuning, before deciding on one from PreciseRF.
The antenna that I selected in February 2020 was the PreciseRF HG-1 WR Magnetic Loop Antenna. The HG-1 WR has now been discontinued, but is very similar to the HG3 which replaced it, although the HG3 has a stepper motor and more sophisticated automatic tuner options. The HG-1 WR is a portable unit with a coax loop, so it can be packed up into a small bag. It has a remote tuner and as a bonus, a rotator, since an MLA has a “null” perpendicular to the loop. It can be useful to rotate the antenna to block noise or interference. It covers 10m-40m at up to 45w, with additional plug-in capacitor modules for 60m and 80m, limited to 10w. According the marketing material, “it is outdoor deployable, water-resistant and ideal for HOA and other fixed installations.” (I’ll have more on this later…)
I ordered the “Deluxe” Package, which included:
- HG-1 WR water resistant tuner/resonator
- AR-1 remote rotator
- HG-2 universal controller
- LMR600 10′ low loss radiation loop & copper induction loop
- PVC three section mast
- 12′ 50Ω feed line
- 12′ CAT6 control cable
- RJ45 T adapter
- T-1U Universal Tripod
I won’t include pictures of the other items, as they are fairly standard coax and CAT6 “ethernet” cable. The coupling loop uses a BNC connector, common for QRP radios. I bought an ABR BNC-PL-259 RG8x coax cable so that I can connect to a PolyPhaser lightning arrestor in my entry panel. The remote tuner and rotator connect with standard CAT6 ethernet cable, so I was able to make a cable that runs about 75′ from my station in the basement out through the entry panel. The T-1U Universal Tripod turned out to be a photographic light stand from Neweer which sells on Amazon for less than $30, rather than $80 from PreciseRF, although their version includes a nicely made adaptor to mount the antenna on the rotator.
Assembly is pretty straight-forward, and well documented in the materials from PreciseRF. Setup the tripod, and screw the threaded adaptor into the base of the rotator, which clamps to the tripod. Assemble the three sections of PVC mast, and attach to the top of the rotator with a pin. Unless you fabricated your own mount, there is no way to mount the antenna without the rotator, at least with the “Deluxe” package. The enclosure with the tuning capacitor mounts to the base of the mast with a couple of thumbscrews. Then the LMR600 coax loop is attached to both sides of the tuner, and held by a clip at the top of the PVC mast. The copper coupling loop clips to the mast, and is connected to the transceiver with a BNC connector. There are not explicit instructions about positioning the coupling loop. It should overlap with the top of the loop, and you may be able to make fine adjustments to the SWR by adjusting the position of the coupling loop. The rotator and tuner connect to the controller with an ethernet cable, and a “T” connector is included to connect both at once.
Once assembled, you can use the controller to tune the antenna to resonate on the desired frequency. The controller can be powered by a 9v battery, or with the included 9v power supply/transformer. I believe that the rotator only works with the AC adapter, as it draws too much current for the 9v battery. The controller has a knob to adjust the speed, and up and down pulse buttons to change the tuning capacitor, or turn the rotator. The approach recommended in the instructions is to tune until you have the highest noise level on the frequency you are tuned, or to use the SWR meter on the transceiver while sending a CW tone at low power. I found this method to be rather difficult, since the bandwidth is no narrow, and the tuning tends to jump up and down a bit, rather than gradually.
After a month, I started to notice that I couldn’t hear any difference in the noise level, so I ordered an antenna analyzer. I picked up a Rig Expert Stick 230 in March 2020. This a fabulous device — small, lightweight, easy-to-read e-paper display, and a long-life rechargeable Li-ion battery. The best part, though, is that it also has USB and BlueTooth, and can be connected to the AntScope2 application on the Mac, Windows, Linux, iOS devices, or Android. This allows you to see an SWR graph, so that you can see exactly where the lowest SWR is found. This has made tuning the magnetic loop much easier.
I will also note that in my case, the resonant tuning changes throughout the day as the temperature changes. I would imagine that the spacing between the plates in the capacitor varies as it heats or cools. I find that I have to retune every couple of hours to stay tuned to the right frequency.
Unfortunately, what I found in April 2020 was that the loop didn’t show resonance on any band, and I couldn’t tune to less than 15:1 SWR! In fact, turning the capacitor had no effect on the SWR reading, so I was concerned that the control cable was bad. I took the antenna analyzer and controller outside to the antenna to bypass the coax and CAT6 cable that I ran, and connected directly to the MLA tuner. I could hear the tuning cap motor running on high speed with the controller, so I could tell that it was working.
I decided to open the cover on the tuning box to inspect it, and about 1-2 tablespoons of water came out when I removed the cover! There were visible drops of water on the top end of the capacitor, and the upper plates, so it appears that water entered through/around the banana jacks on the top of the case. The banana jacks for the external resonator capacitor module were supposed to be “water-proof” but seemed to have leaked when some spring snow melted. Although, in retrospect, it could also have been condensation that formed on the top and dripped down, as well…
To ensure that the antenna would be weather-resistant when I set it up outside, I had applied some silicone caulking around the base of the coupling loop, to make certain that water didn’t enter and get into the BNC connector, and also around the SO-239 connectors on the sides of the tuning box for the secondary loop. I wrapped all of the coax connections with 3M Temflex 2155 Rubber Splicing Tape and Scotch Super 33 Plus Vinyl Electrical Tape for UV protection. I also sealed around the top edge of the lower cap on the rotator with silicone caulk where it looked like water could enter.
I also found a solution to weather-proof the RJ-45 connections. I have had security cameras that use RJ45 with PoE, and found them to be extremely difficult to waterproof. I found the “SockitBox” on Amazon for $7.77, which is intended for outdoor electrical connections like holiday or garden lighting. They make a smaller unit, but it only has two cable entrances. This one is larger than needed for the Y adaptor, but has slots for up to 5 cables.
When I discovered water in the enclosure, I reached out to PreciseRF, and they made some recommendations on drying out the capacitor, etc. In the end, I still wasn’t getting the specified capacitance, so they gave me an RMA to return the tuner for exchange at the end of April 2020. In addition to the weather-proofing precautions that I took the first time, PreciseRF also suggested that I consider mounting the tuning box “upside-down” with the banana jacks at the bottom, thinking the “cable gland” around the CAT6 cable might be less prone to leaking. I did this, but found that an unintended consequence was that it puts the external resonator module too close to the metal shaft in the rotator, and I was unable to tune better that a 5:1 SWR on 80m. I was willing to live with this limitation if it kept the antenna working.
All was well until the end of September 2020, when I again discovered that I was unable to tune the antenna. This time, it didn’t appear that the controller was able to turn the capacitor to change the tuning, as the SWR showed reasonably low, but was a few KHz too low on 20m for the ft8 range. Working the Up and Down buttons on the controller did not change the tuning… I opened up the tuning box, and didn’t find any water this time, but definitely the signs of moisture…
There has obviously been moisture trapped inside the enclosure, which has rusted the (previously silver) barrel of the electric motor to rotate the capacitor, as well as visible corrosion on much of the other metal hardware. There is also some general green discoloration on the inside of the enclosure, which makes me think that there was moisture which steamed some of the green dye from the circuit boards.
I reached out to PreciseRF again, and they begrudgingly provided me with an RMA number to return it to them. They did agree to replace it again, but have already told me that they will not replace it again, even though the twelve-month warranty should go through February 2021.
We received your HG-1 tuner box and have accessed the damage to it.email from PreciseRF
It looks to have been left in the elements, in water for some time. There is nothing usable in this box. With that said Roger has agreed to send you an all new box. However, he notes that he will not replace this unit again. It is a water resistant box not a waterproof box and it will not withstand being left in the elements like this without additional weather proofing done by you. If you want to seal it up totally and leave it out in the weather it will be up to you if it withstands that.
The antenna has been used for “outdoor deployable, water-resistant and ideal for HOA and other fixed installations” and has been mounted to the top of the provided tripod, about 4′ off the ground. Denver is classified as a semi-arid desert, and had only received 7.41″ of precipitation for the entire year up to this point, and had not had any measurable precipitation for the past 60 days. Denver averages only 9-15% humidity during the summer, so it’s not like I live in a tropical environment. I had done as much additional weather-proofing as I thought possible.
I’m now on the third unit, and have weather-proofed as before. I have gone back to mounting right-side up, but have cut a clear plastic cover from some packaging material to shield the banana jacks on the top.
So now I have a dilemma — since PreciseRF will no longer honor the warranty, then if it becomes damaged again, it will be on me to repair or replace it. I have considered having to take it down when not in use, and putting it out again every time I want to use it, which will be a pain. I have contemplated trying to install it in the attic, which would make it difficult to use as a portable antenna in the future.
When “Water-Proof” is not “Weather-Resistant”
I have also been doing some additional research on the moisture problem that I’ve been having, with some input from a fellow ham who is an engineer with a satellite communications company, who is familiar with outdoor equipment enclosures. I found an interesting article explaining how a sealed “water-proof” enclosure that is subjected to extreme or rapid temperature changes can become pressurized in the heat, and then creates a vacuum when it cools, which can actually draw in cool, moist air. This is likely what has happening with my MLA tuning enclosure. While we don’t have much precipitation in Denver, we do have significant temperature swings somewhat frequently. This past week, we went from a high in the mid-70s to an overnight low of only 30º when a cold front passed through. Over Labor Day weekend, we had a similar event where the temperature dropped from 92º during the day, to only 29º overnight.
The recommended solution for this situation is actually to vent the enclosure, which seems counterintuitive. Venting allows the temperature and pressure inside of the enclosure to equalize with the ambient conditions outside, eliminating the pressure and vacuum, reducing any condensation and allowing any moisture to evaporate. While the ideal vent device would probably be one of the GoreTex or ePTFE vent plugs as described in the article, I was unable to source one retail. I was able to get a NEMA enclosure vent, pictured below, which I have already installed on the cover panel of the tuning box, before realizing that the control board was for an HG3, which will not work with my HG-2 controller. PreciseRF had to replace that unit with the correct model. I was able to swap the covers, and keep my newly vented enclosure cover intact.
I would expect that the same situation could occur if the magnetic loop antenna where installed “indoors” in an attic space. Even though the assembly would be protected from precipitation, it would still be subjected to the extreme temperature changes, which could cause cool, humid air to be drawn into the sealed enclosure in the evenings.
I will also note that this exact scenario could apply to the newer PreciseRF HG3 magnetic loop antenna, which is built using the same Polycase enclosure. I don’t believe that this is a failure of the enclosure, per se, but rather the wrong “use case.” I don’t doubt that in controlled conditions, the Polycase enclosure could be submerged in a bucket of water without leaking. The bigger factor is the temperature change.
Have you have ever heated a plastic container in the microwave and had the lid seal when you take it out, being drawn in by the vacuum as the temperature drops, deforming the container? This is exactly the same effect that the weather has on an enclosure like this with the heating during the day, and rapid cooling at night when the sun goes down. When the enclosure is rigid, it has to equalize the pressure/vacuum by pushing through the gasket or seal. This effect can be eliminated by venting.
In addition to the article, I also found an interesting demonstration of the vacuum created by an unvented enclosure during a rapid temperature change on YouTube.
Below is the NEMA enclosure vent installed on the front cover of the enclosure for the tuning capacitor:
The vent is like a miniature “dryer vent,” and will be oriented with the opening downward when the antenna is setup. This should allow for a free exchange of air, equalizing the temperature and pressure, reducing condensation, and allowing for evaporation of any moisture.
I have shared this information with PreciseRF, hoping for two things: first, acknowledgement that I have done nothing to abuse the device which should void the warranty, and second, that they might share this information with other HG-1 or HG3 owners who might experience the same circumstances. So far, PreciseRF is steadfast in their conviction that the Polycase enclosure is “waterproof” and that any additional weatherproofing is the responsibility of the user…
Despite the challenges that I have had with the PreciseRF HG-1 WR Magnetic Loop Antenna, I have been pleased with it overall. I have worked 48 states, Canada and Mexico, making over 750 contacts on ft8 with only 45w power. It is small and portable, and working as expected, except for the water/moisture incursion. I am hopeful that venting the enclosure will eliminate this issue.
I was also very interested in the HG3 when it was announced, as it offers a microprocessor-based controller with an SWR bridge which can automatically tune the antenna to the frequency in use. I could have upgraded mine to the HG3, but it would have cost as much as the original purchase price. Since I’m mostly interested in the digital sound card modes which operate in a fixed frequency range, I decided that it wasn’t worth another $800 to have the auto-tuning feature.