Comet HFJ-350M
Richard Constantine G3UGF gets hands-on with the Comet HFJ-350M ‘Toy Box’ antenna.
Ifirst encountered the Comet HFJ350M compact antenna system while reviewing accessories for Icom’s latest QRP transceiver and mentioned it in the article. It could be seen in photographs showing an aftermarket side-mount antenna bracket for the IC-705. It helped to shift the centre of gravity of this prone-to-tipping radio, before an OEM desk mount was released, and made the radio’s screen easier to see.
I could only see part of the base coil but thought it looked rather robust and better than a similar one I had in my collection. I was unable to find a UK importer at that time to verify this. I could see the antenna in QST magazine and toyed (no pun intended) with asking my son to ship one from the USA.
The moment passed because, shortly after my review appeared, I noticed that the HFJ-350M was now being imported and so placed an order. Well then, what is it, what’s in the ‘Toy Box’ and does it work?
On Arrival
It actually arrived in a reusable, roll-up and stowable plastic pouch. There’s no box involved, maybe it got lost in translation?
The design follows a long line of traditional, base-loaded, tapped coil, mobile and portable antennas that have been around for almost as long as radio itself. In this case, scaled down for use with portable rigs. I obtained a similar one when Yaesu’s FT-817 burst on to the scene, more than 20 years ago.
In general, base loading isn’t the most efficient type of radiator but is the most practical. Additionally, the antenna is a mere fraction of the real thing. You simply can’t expect, as the old pre-metric saying goes to, ‘get a quart out of a pint pot!’
The small size, relative to frequency, is the biggest downside. That is to say, it become less efficient the lower in frequency you go. I wasn’t expecting miracles unless the 10m band was wide open!
Construction aside, how well small antennas work relies on getting a number of things right. Effective earthing or counterpoise system, matching, location and of course the vagaries of band conditions. Nevertheless, this type of antenna can still surprise given half a chance. Mounting one on a biscuit tin in the back bedroom is most definitely not a good idea.
Mechanically, this version comprises a PL259 terminated, 20mm diameter, 47wH, tapped base coil. It provides coverage of 7-50MHz according to the position of the mini wander-plug lead, used to short circuit unwanted sections of the windings. The preferred tapping points are clearly marked, making life very easy with no guessing.
There are two fixed value, additional coils that can be mounted in series above, as and when required. All of the coils are covered by very attractive, heavy duty and durable looking shrink-wrap.
There’s an add-on 15mm diameter, 210mm, 132wH coil for 80m and a further slightly larger diameter, 20mm x 250mm, 410wH inductor for the 160m band. The kit can be obtained without the optional 160m coil for a reduced price, if preferred.
The coil sections are topped off by a satin finish, 1.26m telescopic radiator, adjustable for best VSWR.
The telescopic radiator is also intended to provide coverage of 6m with the base section coil installed but shorted out.
Here a word to the wise and from bitter experience of my youth; never be tempted to close a telescopic whip by pushing it down from the top end. It’s so tempting but, if a section jams, the whip bends irreversibly and you’ll need a costly replacement.
Each additional coil section makes use of the now ubiquitous 3/8in UNF threaded fittings for interconnection, something that could prove useful for further experimentation. I love to mix and match antenna parts from time to time.
The literature stated that the inductors are max power rated for 100W peak SSB. However, this information is modified by the following recommendation: 7-50MHz 100W; 3.5MHz 75W average; 160m 50W average. One to watch if you’re not using a QRP rig.
As mentioned, the base coil is terminated with a fixed PL259 connector and there’s a handy little earth tag affixed to it by means of a screw, nice touch. I considered using the earth tag to connect to a terminal block with multiple counterpoise wires attached. However, as the radio was bound to be close by, connect the earthing system directly to the radio itself seemed preferable.
The manufacturer’s single page leaflet gives brief suggestions regarding counterpoise systems and made me smile. It recommends the following wire lengths in metres with the added rider that, ‘depending on conditions, longer lines may be necessary’: 1.8MHz: 40m; 3.5MHz: 20m; 7MHz: 12m; etc.
Ironically, if I had those quarter wavelengths of wire with me when /P, perhaps I would use them as antennas,
Photo 1: The Toy Box kit. Photo 2: Antenna and mount system with, Elecraft KX3.
Photo 3: Close up of base coil and mount, with Elecraft KX3. Photo 4: Antenna clipped to backpack with Auto-ATU and Icom IC-705. Photo 5: Antenna directly mounted on MFJ manual ATU and FT-817.
rather than counterpoises? Realistically, that’s not always possible or practical in holiday accommodation, on campsites or when backpacking plus not so easy for band hopping, if just listening, etc.
In Use
As any conscientious reviewer would, I decided to spend a day taking the manufacturers at their word, checking out every band with the appropriate counterpoise. Yes, this did include 40m of wire. Results were interesting but, in a nutshell, not as simple or RF docile as anticipated.
I did expect to find the antenna to be very narrow-band with a VSWR of a few kilohertz due to its size and so it proved. VSWRs of less than 2:1 became spot frequencies on the lowest bands. It became clear and very quickly that the VSWR changed to unacceptable levels, caused by proximity capacitive effects, when either I or the surroundings moved or changed.
Following some considerable experimentation, the solution was the most obvious, a genuine earth connection with a short wire lead.
I experimented further, adding counterpoise wires to the earth system. Small, and seemingly positive improvements in reception were noted on some lower bands. Comet recommends the use of an antenna analyser to set up the antenna for best match and, of course, the literature mentions their own model.
Being somewhat old school and for the benefit of the average radio amateur, I wanted to see if I could match the antenna by the traditional VSWR meter method. Result: it can be done but it takes time and patience. Move frequency, band, location, or a quick listen elsewhere and you have to start again. Therefore, using a modern pocket analyser is a great help and can be done without endangering the transmitter.
That said, the quick and easy solution is to use an antenna tuner, easily my preferred option. While the radiator might not just be spot-on resonant it’s definitely the quickest and simplest way to use this antenna, while protecting your investment.
Once tuned manually or auto, the difference on receive is a striking improvement
and bodes well for transmission. Elecraft’s KX3 transceiver has its own internal auto tuner but other popular radios such as Icom’s IC705 and Yaesu’s FT-817 don’t.
Here a word about connecting this or any other compact antenna directly to the antenna socket on a radio. In my experience and having replaced a few sockets, not a good idea. Certainly not recommended if there is no internal tuner, for the reasons I’ve already described.
All of the above radios require an elbow connector for a vertical antenna. BNC adaptors swivel and are a nuisance to keep aligned vertically without additional support. PL259s (FT-817 rear) have locking teeth but, importantly, all arrangements place unacceptable strain on the radio’s output socket with a heavy HF vertical.
Which leads me to the next dilemma. The system doesn’t come with any form of mount. A few frustrating hours were wasted trying to fabricate something and experimenting with mag-mounts. The simplest solutions are sometimes the most obvious. What I eventually came up with can be seen in the photographs. A flat alloy bar and a small hobby type G-clamp, both relatively light and compact to transport and for which I claim no originality.
With the addition of a car boot-lip mount it allows the antenna to be clipped to a wide variety of structures, such a picnic tables or seats, either horizontally or vertically. I really like the Q80 mount normally found on my backpack as it can be quickly unclipped and re-locked again with a spring-loaded lever. A simple alternative would be a drilled metal strip with its own SO239 mount and cable.
But Does itWork?
At the start of this review of my experiences I mentioned three criteria, earthing, matching and band conditions. Having shared with readers experiences of the first two, now under the heading of, band conditions, does the antenna work?
In general, the answer is, ‘Yes,’ with some qualification. While I’m personally no lover of the plethora of contests that overly dominate the bands and hasten the death of conversation, they are a useful tool for checking out antennas. Ignoring the irrelevant 59 sent from a logging computer, would my signals be heard and how weak a signal could I call and still get a report?
From a good clear location and presumably helped by the setup of the stations contacted and, of course, propagation, I was able to get some replies from S5 European signals, when in the clear, no pile-ups busted! Again, the higher bands were better and for now I’m guessing that using FT4 or FT8 data modes will produce some interesting results.
To summarise, this little antenna system has much ‘play value’. It’s ideal present material and definitely falls into the category of doing more with less. The inductors are built to last, making them ideal and improved replacements for my earlier version. The complete kit has found its way into my ever growing ‘Toy Box’.
At time of press, the HFJ-350M is available from Nevada Radio. The 1.8 to 50MHz version retails at £149.95 while the 3.5 to 50MHz versions sells for £129.95.