A COMMON MODE RADIATOR USING THE REZ RECON 40
- Larry Cottrill, K0WUQ
- Aug 26
- 14 min read
Updated: Aug 27
If I'm ever going to go portable, Ease Of Use is a top priority. The system I worked out with this unit gives a vertical antenna that uses no radials, no baluns or other transformers, and no tuner at the antenna or the transmitter. This setup is pure simplicity, because, like all the antennas I've built, it uses a large part of the feed line as the Common Mode Radiating Counterpoise. If you’re not familiar with this concept, see the final subsection of this article. In the case of verticals, I despise radials, especially ones lying all over the ground. To me, carrying a roll of radials out to the field and spending time setting them up 'just so' is patently absurd. I also believe (without proof) that my single, slightly elevated CM radiating counterpoise gives better performance than any radial field on the ground. It does add some directionality, with a horizontally polarized signal from this quasi-horizontal portion of the antenna.
So . . . Why Bother With This Design?
Once this is properly set up, what do we have? Here are the commonly asked questions, and my responses:
Is this in reality a complete dipole?
Yes
Does this function as a vertical with a single, tuned radial?
Yes
Is there full 1/4-wave radiation?
On 20, 15 and 10M: Yes – On 40M, the antenna is shortened by a built-in loading coil
Is there horizontally polarized radiation?
Yes, though some is absorbed by the earth
Can it be tuned to cover the whole band with low SWR?
Yes
Can it be used for different bands?
Yes, by simply changing the counterpoise length and shortening the whip - 40 / 20 / 15/ 10M (and higher, if desired)
Can it be used for DX operation?
Yes – the 20M version got me two DX QSOs the first week: State of Iowa, US (home QTH) to Victoria, Australia (100W SSB phone, RS 57 or better) – and many DX QSOs since
Does it need any form of conditioning (transformer, balun, choke, etc.) where the coax hooks up to the antenna?
No
Are radials needed?
Absolutely not – they would be counter-productive if used!
Is grounding required at the antenna?
No
Is any type of tuner needed, either at the antenna or at the transmitter?
No
Can it be used for high power rigs?
No (I estimate about 200W maximum with modern coaxial types)
Does it deliver harmful common mode current back into the transceiver?
No, if properly choked as shown below
Here's my first cut at a portable station using this antenna – the Yaesu FT-891, powered and grounded using the mighty 2004 Lexus sedan. The second photo clearly shows the important ‘shack entrance’ CM choke – in this case, right up against the back wall of the transceiver. This choke is wound on a 6ft coax jumper, connected directly to the counterpoise coax. An aluminum biscuit sheet is used to protect the vinyl table top from the heat of the transceiver heat sink. The station is simplicity itself:
There are two counterpoise feed lines – a 50ft section for 40/20M and a 20ft section for 15/10M; these will be fully described later.
OK, So What’s the Overall System Configuration?
When I saw the REZ Antenna Recon 40 reviewed in QST (April 2025, p45) I realized that this would be a simple and effective way to achieve portable operation with a CM Radiator antenna design. It would give me the HF bands I was interested in: 40, 20, 15 and 10 metres, in a way that would be portable and easily set up almost anywhere, driven by a simple modern transceiver (I chose the Yaesu FT-891, but your favorite portable/mobile transceiver will do). Here’s my schematic drawing of how I did this for 40 and 20M, using a standard 50ft length of RG-8X coax – the coax is simply flipped end-to-end to achieve proper counterpoise for either band:
Similarly, a standard 20ft length can be used in a 15/10M setup – again, the CM choke is positioned at the correct counterpoise length by simply reversing the coax. The lengths indicated for the vertical whip are my original measurements; they will be adjusted slightly in the field to achieve the best possible SWR for each band:
Using the REZ Recon 40 Base
Though traditionally home-building all my antennas, I decided to splurge on the convenience of this unit - at a mere $300 US, it isn’t pocket change for most of us. But, what you're buying is "lasts a lifetime" quality and ruggedness, plus extreme ease-of-use. I set up for my initial experiments (extremely satisfying at 20M) with the unit configured as a ground-base vertical, with a 17.5 ft whip screwed into the top fitting. This is easily accomplished by attaching the small black screw-on insulating block at the bottom of the unit. For a CM Radiating vertical, avoiding any ground connection at the bottom is crucial!
But as most of us know, getting a vertical up off the ground delivers much better performance. I dug out my old Sunset Channel Leg Tripod left over from my amateur filmmaker days, reasoning that this would be perfect because of its elevated tilting head and almost 6 foot maximum height. However, there was a problem: The bottom of the Recon 40 (and its insulating block) has a 3/8-24 male stud for mounting, while the platform on the tripod has a 1/4-20 male thumb screw to fit the bottom of a camera. So, I rigged a hokey male-to-male adapter, which worked to mate the two. However, when a wind storm blew the antenna and tripod over one afternoon, the threads on my adapter stripped out – the spindly adapter was obviously a weak point.
I reasoned that the connection would be much stronger if the large area of the bottom insulator could make full, tight contact with the tripod platform. Consulting with Mike Giannaccio, W5REZ – the REZ Antenna man himself – he pointed out that the 3/8 male stud in the insulator could be removed and replaced with a tiny 3/8-24-to-1/4-20 threaded insert - I had such an insert already in hand! Mike warned that the stud was threaded in with a locking compound, but I found that Vise Grips (TM) broke it loose fairly easily and I removed it. This allowed me to thread in the insert (equipped with its own locking compound), using a short 1/4-20 bolt to thread it down flush with the bottom surface of the insulator – voila! This allowed a rugged attachment of the Recon 40 to the tripod platform:
Setting Up the Coaxial Counterpoise
Each counterpoise / feed line is made from a standard finished RG-8X coax section - a 50ft section for 40 and 20M, and a 20ft section for 15 and 10M. I used my ‘standard’ ferrite cores to fabricate the chokes: 4-inch (105mm) OD x 3-inch (76mm) ID x 1-inch (25mm) thick Mix 31 toroidal cores. The longer coax needs a choke that will effectively block 7 MHz RF – for this, I use 12 turns wound comfortably tight on the core. The shorter line needs to block 21 MHz, and I use 9 turns wound on an identical core. I just secure the windings with a strong cable tie where the coax first passes through the center and another tie where it finally comes out the other side. These are not over-tightened, so the exact position of the choke on the coax can be adjusted for initial tuning. Smaller cores with more turns could be used, of course, these are just my preference.
The chokes were located on the counterpoise / feed lines as follows:
50ft counterpoise:
Choke is 31 ft from one finished end, just under 15 ft from the other end, with almost 4 ft taken up by the winding on the choke (12T)
20ft counterpoise:
Choke is about 11.5 ft from one end, about 7 ft from the other end, with almost 2 ft taken up by the choke winding (9T)
I had already come up with a way to arrange the counterpoise / feed line in a portable setup. It’s highly advantageous to keep the CM radiating portion of the coaxial line above ground. To do this, I use plastic ‘step-in’ posts designed for temporary electric livestock fencing. These are only about one metre tall, but that’s a lot better than having the radiator coax on the ground, where most of the RF power would be lost! The posts are lightweight and easily carried, and only a few are needed. To use them, a few cable tie rings are provided on the coax line – not tight, just made small enough to not fall off over the end fittings:
These simply snap onto the topmost lug of the plastic posts – and, they can be easily removed when you’re ready to pack everything up at the end of the day. Since they aren’t pulled tight around the coax, you have a sliding fit so you don’t have to worry about setting the posts at exact locations on the site. I make one exception to this: I anchor the crucial CM choke to one of the posts, using a couple of cable ties pulled fairly snug to hold it. This is done in such a way that the coax wound on the choke is still able to be adjusted for fine tuning – but, this really only needs to be done once and you’re finished with it.
Basic Procedure - Setting Up In the Field
Once you’ve got the antenna support and the coax cables figured out, all that’s left is to take it someplace and build your station, more or less like this:
Stake out your location. In rare cases, you can set up close to a shelter house or some such where you have access to a 120 VAC outlet – lucky you, you can use your power supply (if you can keep it safely out of the weather); otherwise, set up your battery or vehicle for DC power.
Set up your transceiver in the station location, connect to DC power source & ground
Your antenna location can be anywhere within about 60 feet of your transceiver, because you can always link both of the cables in series, with a barrel connector!
Set up your antenna support and attach the Recon 40, with the whip adjusted for the band you intend to use.
Attach the proper end of the correct cable to the Recon 40:
40M: Attach the long end of the 50ft cable to the Recon 40 – be sure the switch on the Recon 40 is set with the loading coil un-shorted
20M: Attach the short end of the 50ft cable to the Recon 40 – switch must be set to coil shorted position
15M: Attach the long end of the 20ft cable to the Recon 40 – switch must be set to ‘shorted’
10M: Attach the short end of the 20ft cable to the Recon 40 – switch must be set to ‘shorted’
If needed, attach the other cable using a barrel connector
At the transceiver end, attach the ‘shack entrance’ choke stub between the cable end and the transceiver ANT connector
Go to the choke position(s) on the coax, and step the plastic post into the ground – you should allow a little slack between the antenna and the first CM choke
Add one or more plastic posts evenly spaced between the antenna and the first CM choke – use a wire tie ring to attach the coax (sliding fit) at the top of each post
The remaining feed line can be supported on posts, or can just run along the ground until you get to the transceiver location
Temporarily disconnect the coax at the transceiver end. Use your RigExpert or nanoVNA to adjust the final length of the whip so the lowest SWR is at the middle of the band (or wherever you want it) – if SWR is completely wild, you have mis-set the shorting switch on the Recon 40!
When adjustment is complete, re-connect the coax at the transceiver – you’re ready to blast away!
If you use an antenna support with a tilting head as I did, tilting it down makes length adjustment of the whip super easy:
You’ll probably have to tilt it back up vertical to see the exact SWR after each slight adjustment. It’s convenient to have a helper tilt it, make the adjustment and raise it again while you observe the changes in SWR. If possible, measure and record the actual length you finally use for each band, so you can simply set it to the measured length the next time out.
That’s about it for setting up. Changing bands is just a matter of lengthening / shortening the whip, making sure the coil shorting switch is set right, and setting up the right coax length at the antenna. Once you’re used to it, the change can be done in minutes.
Once setup is complete, you just use this antenna like any other dipole. All the usual cautions associated with portable antennas apply. This includes being especially watchful for people and animals contacting the whip while you’re transmitting; and of course, you’ll want to try to locate the coax run where it’s not likely to be in the way of human traffic.
The final section is for those unfamiliar with RF Common Mode creation and the radiation that results from it:
Common Mode Current and Common Mode Radiator Design
Radio frequency interference (RFI) is usually blamed on the phenomenon of Common Mode (CM) current present on the coaxial feedline when transmitting. Unchecked, it can wreak havoc in the ham shack, and in the home or structure electrically powering the station. Amateurs that are aware of this phenomenon go to great lengths to block CM current from ever getting close to their station equipment. It can drive the everyday appliances in modern ‘smart homes’ crazy.
So then, why try to use it as part of a transmitting antenna?
Well, first, you have to understand where it originates. “All antennas are dipoles” – this is probably actually true for antennas that aren’t some form of ‘loop’ design. All this truism means is that in order to work, an antenna has to be driven as two separate sections that are supposed to be electrically ‘balanced’. A perfect dipole would be two identical wires in-line with each other (and almost touching at a central point) in outer space, both wires being the perfect length to resonate at a radio frequency we want to use. If a perfectly balanced feedline could be connected to the center of such a dipole and we drive RF at that frequency through the feedline, the antenna will radiate virtually all the RF energy it gets, and there will be no CM current pushed back onto the feedline from the antenna. The entire antenna system would be perfectly balanced in such a case.
In practical reality, we can’t produce an antenna with any such perfection. A real antenna will have some impedance difference between the two sides. Any such difference will cause some of the RF energy to be diverted back onto the feed line as a ‘common mode’ (CM) current between the antenna center point (the ‘feed point’) and the transmitter end of the line. Unless the line is very short, this will form a standing wave – just like the standing waves on the elements of the dipole – and that standing wave will radiate that frequency in proportion to the amount of CM current (which can be measured along the feedline, with simple instruments).
The most common case, using coaxial cable, is the simplest to describe. In this case, the RF currents inside the coax will still be perfectly balanced: two currents 180 degrees out of phase at every point along the line. For Laws of Physics reasons, any CM current present will be relegated to the outside skin of the coax shield, whether the shield is foil, braid, semi-rigid tubing or whatever. Essentially, at the feed point where the coax ends, some of the internal shield current can’t make it onto its half of the dipole, so that fraction of the current ‘turns the corner’ and ends up on the (relatively low impedance) outside of the shield! This CM current can (and must) be dealt with completely separately from the internal coax RF currents. This why a ‘perfect’ SWR of 1.0 does not guarantee that no CM current is present! The internal reflection that affects SWR has no bearing on the external CM current, and vice versa.
When I first learned of the existence of CM current and its separate existence on the coaxial feed line, my first thought was, “Hey – I could use that!” I actually thought this was a brand new, radical idea – I learned later that this had been shown to work back in 1948! Even later I learned that it was often used in a limited way as part of some ‘modern’ antenna designs. But I didn’t know that at first glance, so I proceeded boldly forward, hoping for the best.
My basic reasoning was, everybody ends up dealing with some CM current that’s naturally occurring and un-wanted – why not force ALL the RF current from the shield side of the coax back onto the exterior of the feed line, and then substitute that CM current for one entire leg of the dipole? The principle seemed like a shoo-in. Uh, but wait . . . there’s one little flaw: For this to work, there has to be something that forms the ‘end’ of that leg at the quarter-wave point so that the correct standing wave will be created at the desired frequency.
The cutoff end of one dipole leg forms the standing wave because an open end (1) blocks the RF current from going further; and (2) reflects the wave energy back toward the center. I can’t literally cut the coaxial feed line at the right spot, because it has to carry the RF current from the transmitter to the feed point. What to do? Well, fortunately, I had noticed that there was a construct (and a very simple one) that has been used for decades to stop CM current in its tracks – the coaxial CM choke! I knew such a choke effectively blocked CM current from getting back into the shack, because that’s exactly how everyone was using it. But, did such a choke cause the necessary reflection to sustain the quarter-wavelength standing wave required?
I built my first test of this as a 40M Inverted Vee using the feedline and a big CM choke as one end of the Vee – I call this the ‘near leg’ with the plain wire end being the ‘far leg’. At the center, there was just a ‘barrel connector’ (so the coax could be simply attached) where the shield of the coax simply . . . ends! So I had an Inverted Vee that looked like an end-fed antenna but was actually a 50 ohm center-fed dipole – with no ‘third leg’ of coax hanging vertically from the center! And the horrible thing was, it worked!
And so, that’s what all my antennas – even verticals – are like: Your coaxial feedline runs out to a feed point, where the shield simply stops; you continue the center wire in the form of a quarter-wavelength wire (or vertical tube, or whip, or whatever); and then you go back from the feed point a quarter wavelength along the coax and wind a proper sized ferrite-core choke. That’s all of it, right there. The rest is just the details of mounting and supporting the physical structure. And of course, you should provide another choke as well as a good ground where your feedline enters the shack.
So, what about that horrible RFI that comes inside and ruins everything? I recently made measurements along the entire 20M counterpoise carrying a 100W CW signal to the antenna illustrated earlier. What I found was that the quarter-wave CM choke produces a radical drop in CM current, to about 4 percent of the maximum RF current (as measured at the antenna connection). After going past a second choke, right at the entrance to my shack, the CM current remaining was immeasurably small. Protected this way, this antenna produces no more CM current at the transceiver than any other 20M vertical mounted a reasonable distance from the radio. The unit as shown above should be usable without fear of serious RFI problems. ‘Your mileage may vary’ depending on the size, quality and location of the chokes you provide.
Conclusion
I hope other amateurs – especially those interested in portable operation, such as Parks On The Air (POTA) and emergency operations – will consider the design presented here. Use of the REZ Antenna Recon 40 base unit makes it incredibly simple to implement, with a support of your choosing. With a couple pieces of coax, a couple of ferrite cores and some inexpensive plastic fence posts, an effective and powerful vertical can be set up and put on the air quickly and easily for the 40, 20, 15 and 10M bands (and others, if you want to experiment with different counterpoise lengths!). Go for it!
73
K0WUQ
Comments