A Remote Control Antenna Link
by Dave (KO4YLZ)
Like so many other SOTA activators, I’m constantly tweaking my gear to optimize it for all the variables we encounter in the mountains. One of my main antennas is, of course, an EFHW. Paired with my KX2, I generally get sub-1.6:1 SWR on 40, 20, 15, and 10. However, 30m has an unacceptably high SWR. I considered switching to a multi-link EFHW, but hopping up to change links to chase an inadvertent S2S on another band is inconvenient, especially when the link for low bands is generally well beyond my reach and requires lowering the antenna.
To fully optimize my EFHW I decided I needed to come up with a way to add a “remote” link for 30m. After a couple of design iterations, I came up with the light-weight and compact remote antenna link shown/described below. The version described here is not my final one. While this “pill bottle” version is still quite small, I’d like to reduce the size even further.
Let’s start with the end…
Here’s the final product. It weighs under 1 oz. and is ~2.5” long. I have included a complete parts list at the end of the article.
My design considerations were:
- Small and light
- Range of at least 75 ft.
- Reasonable battery life
- Waterproof
- Durable
- Able to withstand the current at the 30m position on the antenna
- Doesn’t adversely impact antenna performance on other bands
- Easy to connect and disconnect from the antenna
Executive Summary
For those in a hurry, here’s a summary description of the remote antenna link. It consists of a very, very small remote control Bluetooth relay powered by two CR2032 batteries (user supplied). A set of contacts (either normally open or closed) lead from the relay to male and female 2mm bullet banana connectors in the cap of the pill bottle. Two matching bullet banana connectors at the antenna plug into the remote control link.
To operate, you simply open the bottle, switch on the batteries, re-install the cap, extend the antenna, and plug the bullet banana connectors at the antenna into the opposite pair embedded in the cap of the pill bottle. When you are ready to operate, you just push the remote button. When the remote button is pushed, the relay opens or closes thereby making or breaking the link and physically shortening or lengthening the antenna. Below is a picture of both sides of the mini-relay (antenna wire not shown)
The relay can run on 3.5-12 volts and can handle 1A. The included 1-button transmitter sends a signal (455mHz) to the relay and either opens or closes a set of contacts depending on how the user programs the relay.
Wiring It Up
I won’t bore you with a detailed written explanation…it’s a pretty simple build. Below are a simple wiring diagram and a series of photos with build details [click images to enlarge].
Making It work
The relay has three operating modes: latched, momentary, and toggle. Because I wanted the 30m link to be normally connected when the relay is in standby waiting for a signal from the remote control and open and stay open when I pressed the remote button, I had to reprogram the relay. Programming is accomplished by turning the unit on, then putting a 22ga wire jumper between the two “programming dots” and then pressing the remote button until the LED on the relay board flashes the number of times specified in the instructions for the operating mode you want. The instructions are not very clear, so I recommend connecting the unit to your VOM in continuity mode to figure out the button/LED flashing sequence that corresponds to how you want the relay to behave.
Testing
The unit is rated for 1A so, to be sure I wasn’t going to blow it up when I transmitted with 10 watts, I used ChatGPT to produce a very nice graph showing the current at the 30m position on the antenna for each band when transmitting with 10W. Here’s the graph. Luckily, the maximum current at the 30m position (~43’) is .6A at 7.062 MHz.
I tested the unit on my KX2 by setting my operating frequency and then putting the KX2 tuner in bypass mode and pressing the button on the remote control; Ding…the SWR jumped immediately to the expected pre-determined (and acceptable) SWR value for the 30m band!
The unit draws 7mA when in standby and 32mA when the relay is energized. Assuming 40 minutes of operating off of the 30m band (relay not energized) and 20 minutes on the 30m band (relay energized), I calculated that approximately 17mA would be drawn from the two CR2032 batteries. Given that a CR2032 battery has a capacity of 220mAh, they should provide about 12 hours of operation.
Any questions, please comment.
Dave – KO4YLZ
Parts List
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This is such a brilliant idea, Dave! Thank you so much for sharing it with us. Please feel free to share any iteration updates with us!
Cheers,
Thomas
K4SWL
When will you start selling these?
Couldn’t you just use the tuner in the KX2 to get the antenna to work on 30m? I use the tuner in my KX3 to get 12 & 17m with my Packtenna 40-10m antenna. I ask because I don’t use 30m because there’s no voice coms allowed there. Or even still a 9:1 EFRW with the KX2 tuner.
I could, but it’s a tough match for my KX2 and my goal was to have the antenna as close to resonant length as possible using a link given the results of my antenna tuning in the actual very compromised configurarion I usually use. Without the link SWR was ~8:1, with link, 1.2:1.
Gee, just when I thought I had seen it all….. but then my memory kicked in and either SotaBeams in England or some other place had a similar item for a 40-20 meter dipole years ago.
Such ingenuity and a well described show and tell article of how to do it. Even though I can still get up and drop my antenna if needed it is so impressive to see what other hams can and will do to make that one elusive contact.
Thanks for sharing.
Dave-
That’s an exercise in ingenuity- Nice!
I’m no stranger to tiny boards- currently waiting on IF filter PCBs measuring 1.5″ x 0.8″. An upgrade to my homebrew rigs.
72- Dave, K1SWL
Fantastic! This would pair nicely with HB9EAJ Stephan’s 7-band portable EFHW antenna ( just search HB9EAJ 7-Band). I can attest that antenna works well with a pole or a tree support.
Could replace the CR2032 Cells with a small rechargeable LiPo pack ( e.g., out of a discarded vape pen) or a 9V rechargeable battery for longer life.
Great work! 72, Scott VA7SNJ
Clever design, well done.
I wonder how it compares to traps, does it have any advantage over it?
I haven’t built an EFHW with traps so all I can say is that my goal was to get 30 dialed in with a link on my otherwise unlinked antenna that has great SWR (untuned) on 49, 20, 25 and 10, and avoiding having to get up to lower the antenna and connect/disconnect a link. If someone has built a 5-band trapped EFHW that works well with a ln antenna that gets no higher than 15′ at the high end, that I would build
It’s such a great idea! I personally utilize the ATU to make my 7 MHz EFHW work at 30m band. Also I’ve added 110 uH coil and 2-m whip to make it support 80m band without ATU and significant length penalty. The result is super convenient, it allows operation both at 3573 as well as in the beginning of SSB sub-band. It’s good enough for reaching nearby stations even at 10W SSB (up to several hundred kilometers).
Any issues with the Bluetooth transceiver sitting next to the relay and HF antenna? I expect there aren’t, but we’ve, (KO6BTY and myself), heard some things, (on the headphones, not the streets
), while developing with simple receivers and Bluetooth nearby. We had to test several different Bluetooth audio transceivers to find one with low to no noise.
No issues at all CW. Haven’t tested SSB
What’s the availability of a similar receiver that uses a latching relay like QRP tuners use? While you’d still probably want the power switch, that would decently reduce the number of batteries you consume if it’s not worth adding the complexity of tiny lithium-ion batteries. Also having a pair of these could be nice for the close-in contacts of a linked dipole!
I believe that there is a company selling a pair of these that operate from a single remote control that has a two buttons, one for each unit. I just not 100% sure that it is a simple relay as opposed to current carrying switch.
What a great project! Some years back I bought some nice link switches that George KJ6VU (PackTenna and Ham Radio Workbench) to upgrade my rather ancient linked dipole from the New Jersey QRP club and Joe Everheart N2CX. It used alligator clips to add or subtract extra lengths of wire to resonate on various bands. It’s still a very well made antenna with a really excellent type of lightweight springy wire , a very sturdy center insulator and RG 174 feed line. Over time the alligator clips kept breaking off.
In discussions with George he was thinking about using bluetooth to control latching relays to avoid lowering the antenna every time you wanted to change bands, but never developed that. On the actual antenna, what length are you adding or subtracting from your EFHW (assuming it’s a total length of 66’) to get resonance on 30M? Thanks!
Yes, you are correct. I’m “trimming” the antenna by ~ 20 odd feet for 30m band. Since I operate with the antenna abnormally close to the ground (15″ at high end), my antenna trimming certainly doesn’t follow what the straight up math would suggest for resonance on each band.
Very neat idea ! I’m thinking two for a linked dipole
There is a relay trick to limit current which W1FB mentioned in a CQ article on an antenna switch.
The connection to the relay coil is made through a resistor to limit current to the holding current for the relay. In parallel with the resistor is a bypass capacitor to allow enough coil current to turn on the relay when switched on.
Of course circuit surgery would be required on the module in this case.
Dave
Are you just cutting the wire at where the 30m band location; then adding the length that will get you resonant on 40m?
Terry k8tdn
In a sense. Cut to the approximate 30m length and tuned, then added link, and wire and tuned for 40m.