Category Archives: VHF/UHF DX

The technical side of working more DX on VHF/UHF

VHF-UHF Baluns

This is the talk that I presented at the RSGB Convention in October 2018.

RSGB will eventually release a video of the talk, but in the interest of publishing the technical content as soon as possible, you’re very welcome to download and click through the slideshow.

Even without the voiceover, the important points are clear:

  • Today, “baluns” are about controlling interference, reducing noise
  • Choke baluns are best for doing this.

So we should aim to:

  • Minimize common-mode RF currents on the outside of the coax
    and along the boom.
  • Don’t upset the good feedpoint balance that VHF-UHF Yagis already have.

Comments and discussion, please!  This is new information for VHF-UHF, and certainly not the last words to be said about it. It’s still very much ‘work in progress’.
Leave a reply

 

Technical slide content © 2018 Ian White, GM3SEK
Side template © 2018 RSGB
Images as attributed (except where that would be embarrassing).

Switching 24V Relays from a 12V DC Supply

How to operate a 24V relay with no 24V DC supply

It only needs this simple circuit.

Relay pulser topside

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NEW Parts List – see below

Most amateur equipment operates from 12-13.8V DC; but many of the best RF coaxial relays are military surplus and designed to operate from 24-28V DC. Obviously you can generate 24-28V using a switch-mode power supply operating from 12V, and circuit boards are very cheap on eBay. But that isn’t the only way to do it… and not always the best way, either.

“Relay Speed-up” Circuit

This little circuit was originally published in my Radcom ‘In Practice’ column for April 2002 as a way of speeding up the slow open-frame antenna changeover relays that are used in many older power amplifiers. It works by providing a switching pulse at twice the normal voltage, which will roughly halve the switching time. For more details, download the Radcom article.

The idea appears to have originated with K1KP and K6XX. To make it compatible with the switching outputs of most modern transceivers, I added the low voltage / low current switching interface (TR1) so the final circuit looked like this:

rc9181

Corrections: TR1 should be ZVN3306A (or 2N7002) and TR2 should be ZVN3306A (or similar)

Fig.1: Relay speed-up using a pulsed low-side switch.

Notice that the relay(s) are connected to the positive supply rail, and the switching circuit TR2-TR3 is connected at the ‘low side’ between the relays and ground. For that reason, this type of circuit is often called a “low-side switch”.

Alternative Use: Operate 24V Relays from 12-13.8V

This same circuit also has another use: it will allow the large majority of 24V relays to function directly from a 12-13.8V DC supply, with no 24V supply at all! This is because most so-called “24V” relays only need the full voltage while actually switching over. Once the relay has pulled in, most “24V” relays will hold in quite satisfactorily on 12V or even less. So there’s actually no need for a continuous 24V supply – all you really need is that first short pulse.

If a low-side switch is what you need, simply connect Fig,1 to a 12-13.8V DC supply rail and most 24V DC relays will function almost as normal. However, we can simplify it a little more…

Now Even Simpler

Fig.2 is even simpler than Fig.1 was. We have now converted the whole circuit into a ‘high side switch’ which no longer needs a separate control interface. The active switch is connected between the supply rail and the relay; and the ‘low’ side of the relay is connected directly to the ground return. We can now control the relay directly from the gate of Q2, so Fig.2 is simplified to just the two transistors and a handful of other components.

Fig.2: High-side pulsed switch.

Parts List for Fig.2

C1: 220uF 35V, 10mm dia, lead spacing 5mm (eg Farnell 9451293 )
C2: 1000uF 35V, 13mm dia, lead spacing 5mm (eg Farnell  9451323 )
D1, D2: S1M (SMD version of 1N4007, eg Farnell 2675069 )
Q1: TIP32C PNP bipolar, TO220 (eg Farnell 9804161 )
Q2: NDT2955 P-channel MOSFET, SMD (eg Farnell 9846271 )
R1: 1K0  0805 SMD (various sources)
R2, R3: 10K  0805 SMD (various sources)
R4: 100K 0805 SMD (various sources)

CIrcuit Notes for Fig.2

About C1 and C2: C1 is the capacitor that adds an extra 12V pulse in series with the permanent 12-13.8V supply. The value of C1 usually isn’t at all critical, and in most cases 220uF will give a switching time almost identical to what you’d see with a conventional 24V supply. You can experiment with different values for C1 while using an oscilloscope to monitor the pulse shape and voltage. 

C2 is a reservoir capacitor connected across the 12-13.8V supply rail, to help meet the pulsed current demand. If the relay is mounted remotely (eg at the masthead) then the whole circuit needs to be located close to the relay itself, but the presence of C2 at the load end of the supply cable makes it unnecessary to use heavy wiring.

C1 and C2 see no higher voltage than the 12-13.8V supply rail, so a typical voltage rating would be 16V. (Higher voltage ratings are perfectly OK, of course; but not actually needed.)

Do not connect a ‘protection’ diode across the relay coil – it isn’t needed. The switch-off surge is absorbed by recharging C1 and C2.

The whole circuit can be built on a scrap of Matrix Padboard (search eBay.co.uk) or Veroboard. To fill some free space on a PC panel that was mainly for another project, I made a very compact semi-SMD version as shown in the Lead Photo and Fig. 3.

Relay pulser underside

Fig.3: Compact version of the pulsed high-side switch.

 

Page © 2017-8 Ian White, GM3SEK
Fig.1 and related links © RSGB)

13 Ways NOT to Design a 432MHz Transverter Kit

  1. When designing a UHF transverter, start from a 50MHz design. You already stretched it to 144MHz, so everything is sure to stretch a little bit farther. What could possibly go wrong?
  2. Do not leave a continuous groundplane underneath the PC board. Beneath any UHF striplines, be sure to create gaps in the groundplane by routing IF and DC tracks underneath as well. This will ensure that important 432MHz, 404MHz (LO) and 28MHz IF signals are all freely shared around the entire board.
  1. Wherever there is groundplane both above and below the board, take care not to use too many RF grounding vias between them.
  1. Spread yourself out – make all leads and traces nice and long, especially if they are carrying UHF signals.
  1. When using chip capacitors for critical UHF bypassing, be sure to use thin ‘thermal relief’ connections to add some extra inductance between the capacitor and the groundplane.
  1. Even though you are pre-installing some SMD devices in areas of the board that are carrying UHF signals, include plenty of larger, less suitable, wire-leaded components in these areas as well.
  1. If one of your pre-installed SMD devices is a PHEMT with a very fragile gate (like the ATF-34143) be sure to provide a grounding link for the gate. But then require the kit builder to remove this link, to exchange it for a slightly different-shaped piece of wire. This will greatly increase the chances of successful ESD damage.
  1. When ordering overtone crystals, always choose the highest frequency available – in this case, 404/3 = 134.67MHz. Use this crystal in a simple one-transistor oscillator (which specifically requires 9mm long leads on the transistor in order to oscillate) and do the frequency-tripling in the collector circuit of the same transistor. Pass the output through a single tuned circuit at 404MHz and then use the hottest possible modamp to amplify what comes out.

Don’t ever pause to question why everyone else does it differently. Other reputable designers use 101MHz crystals in more complex oscillator circuits, followed by two frequency doublers. But you’re an innovator, so you don’t need to do that.

  1. Do not analyze the gain distribution… or if you do, ignore the fact that NF Analyzer instruments only measure the NF of the transverter itself. It’s always safe to assume that the NF of the following HF receiver will be zero (even if you also happen to manufacture a matching range of HF transceivers with an NF of at least 15dB).
  1. To achieve the best possible gain distribution, do not include a RX IF stage between the transverter and the transceiver. Also do not include an LPF at this point, as it will impede the rich two-way flow of signals and harmonics that is so necessary to produce birdies.
  1. When installing a Mitsubishi PA module, use only a small piece of 1/8in aluminium as a “heat spreader”. Do not use a finned heatsink – the bottom panel of the box will do just fine. Do use a fan, but nothing larger than 40x40mm (choose the loudest possible). In the assembly manual, emphasize the need for washers to separate the mounting flange of the PA module from the underside of the board. Anywhere around the PA module, keep the number of RF-grounding vias to an absolute minimum.
  1. Where metal-to-metal contact is required for shielding panels, be sure to use tough grey paint.
  1. And finally: if someone posts warnings about any or all of the above problems (eg http://tinyurl.com/ydynjb5n), ignore them. On no account change anything in your original design.