Paul Thurst – Page 17 – Engineering Radio

HF VHF receiver diplexer

UPDATE and bump: This post is from eleven years ago, but I have been working on an SDR project using one of the RTL- 2832 chips. I had to make two more of these units, so the prices and part numbers have been updated.

I have acquired one of those broad-banded software-defined radios, an Icom PCR-1000 to be precise, and all is well. I am enjoying listening to various MF, HF and VHF radio stations. However, there is a slight problem. Very slight, almost too small to even mention, more of an inconvenience than a problem. Still, if I am being inconvenienced, then others are too. This issue is with the antennas. My K9AY antenna works wonderfully from 500 KHz to 25 MHz or so. My discone antenna works wonderfully from about 30 MHz all the way up to about 1 GHz. In order to enjoy the full range of the receiver, I need to switch antennas. I have a small switch on my desktop, but it seems inconvenient to reach over and switch it when going from the AM band to the FM band or something similar. Therefore, I have decided that I need an HF/VHF receiver diplexer. One would think that such hardware is ready-made for such instances. However, nothing I could find commercially would do the trick.

Thus, since I could not buy one, I decided to build one to add to my collection of receiver doo-dads and nick knacks. The design is relatively easy, a back-to-back low pass/high pass filter system with a 50-ohm impedance throughout. Something with a sharp cut-off around 30 MHz or so:

Looks pretty good, 5th order Chebyshev filter, perhaps .1 dB ripple in the pass bands if well made. Schematically:

Then it comes down to the building. Since this is going to be used in the UHF range, care and attention needs to be paid to the layout of the components and the design of the circuit board. Some of those capacitance values are not standard, however, by using two capacitors in parallel, one can get pretty close. Since this is going to be used for receiving only, I may be splitting hairs, however, I have found that well-designed and built equipment is worth the extra effort.

The board layout looks like this:

I tried to keep the traces as close to 50 Ohm impedance as possible.

As one may be able to discern, C2 and C3 are in parallel to make 192 PF, C5 and C6 are in parallel to make 60 PF, and C7 and C8 are in parallel to make 163 PF.

The input and output RF connectors are whatever the builder wants to use, however, I would recommend at least BNC or type N for the VHF/UHF side. My unit has all type BNC female connectors. Parts list:

Nomenclature	Value	Mouser number	Cost (USD)
C1	150 PF SMT	581-12065A151FAT2A	0.72
C2	12 PF SMT	581-12061A120JAT2A	0.26
C3	180 PF SMT	810-CGA5C4C0G2J181J	0.27
C4	68 PF SMT	77-VJ1206A680FXACBC	0.59
C5	50 PF SMT	581-12062A500KAT2A	0.41
C6	10 PF SMT	80-C1206C100J2G	0.54
C7	3 PF SMT	581-12061A3R0CAT2A	0.34
C8	160 PF SMT	581-12065A161J	0.34
Case	Diecast, 4.3 x 2.3”	546-1590WB	10.71

I chose a smallish, diecast aluminum case, which matches my other receiver gear. The circuit board noted above is 2.9 x 1.7 inches, which is a little bit small. I used 18 gauge wire between the input/output connectors and the board.

The inductors were made by hand. I used a small screwdriver as a winding form, making the turns tightly and then spreading them out to the proper distance.

Inductor chart:

Inductor	Value (nH)	Diameter (mm)	Turns	Length (mm)
L1	173	8	6	9.5
L2	468	8	10	9.8
L3	414	8	9	8.7
L4	146	8	5	7.2
L5	186	8	6	8.6

The most expensive part was the circuit board, which cost about $16.00. The rest parts were about $22.00 including shipping.

As built photos:

HF VHF diplexor with components installed — HF VHF diplexer with components installed

I have installed this already and it works great. I will need to get the spectrum analyzer out and run some signals through the various ports to see the attenuation and 3 dB roll-off points.

RIP Ray; you were a good guy

By now, most have heard of the passing of Ray Topp, publisher of Radio Guide magazine. I was shocked when I first learned about it in the middle of February. The family has decided to cease publishing the magazine, which is understandable, but also a loss for the industry. The goal of Radio Guide was to provide “real-world technical information.” When I wrote an article, I always thought about the various people I worked with over the years and what they were concerned with.

If you have not read the Radio World article: Ray Topp, Publisher of Radio Guide Dies

The last I heard from Ray was the third week in December. He said he had a bout of COVID and there were some complications. He said he was trying very hard to get the January/February 2023 issue done. I sent in my article in early January but never heard anything back, which is unusual. When the publishing date came and went, I thought that perhaps he was still recovering. Unfortunately, that was not so.

My final article for Radio Guide was to be titled: Learning with the Libra VNA

learning with a Libra VNA Download

Since it was never published, I figured perhaps some might find it useful.

FLX20KFAX50K+HD

Greetings from the Roxborough tower farm, a place with roots. It is slightly northwest of Philadelphia, PA, and is home to many TV and FM stations. The public road that cuts through the tower farm is called Domino Lane because if one tower falls, they all fall. A comforting thought to those that live in the vicinity I am sure.

View from the residential neighborhood next to the site

The reason for the visit; this rather nice GatesAir FLX20 transmitter:

Newly installed FLX20+HD, WRNB Philadelphia, PA

I must admit, I am growing rather fond of these transmitters. This unit is being installed because the station had to move from its old site, just down the hill. The tower owner is taking down the tower and building due to the age of the tower. Thus, it was moved into the KYW-TV building. If Wikipedia is to be believed, KYW-TV is the oldest TV station in Philadelphia, signing on in 1932.

The site is still being built as we were installing this transmitter. These days, the electricians are having supply chain problems like everyone else. There were delays getting the large electrical panel board and other necessary things for the build-out.

Cooling system high point, sight glass, and air purge valve

Overall, the installation went well. This system is using flexible hoses for the coolant loop. We have installed two of these liquid-cooled transmitters with 1 1/2 copper pipe. These days, copper pipe is expensive, so most are opting for flexible hose installation.

Topping off the pump station after 50/50 fillup. After the initial system fillup, it takes a while for all of the dissolved air to come out of the Heat Transfer Fluid (HTF). The extra steps with a liquid-cooled system are worth it, especially if the station is running HD. With the HD carrier on, the transmitter efficiency is 54% AC to RF. With a TPO of around 15 KW, that is a whole lot of heat that needs to be dissipated during operation. It is much cheaper to pipe the heat outside to a heat exchanger than to use several tons of AC to remove it from the room.

Overall, this was a fun project.

Making a notch filter

One small RF project that I am working on; a 770 KHz notch filter. I always figure if I am having this problem, then others may be having it too. This is a relatively simple idea, a resonant LC circuit (AKA a tank circuit) tuned to the carrier frequency. It should have a bandwidth of +/- 15 KHz of the design frequency. Another requirement; use the parts I have available. Finally, the environment in which this is to be used is a high-noise room; with lots of computers, LED lights, etc therefore it needs to have excellent RF shielding.

Something like this would work well for anyone that lives around an AM transmitter site and is having problems with receiver sensitivity or transmitter intermodulation.

The basic design looks like this:

Time for a trip to the local storage facility known as “The Barn.” In my backyard, there is a small agricultural structure that is used for storage of just about everything. In The Barn, I found several parts salvaged from an old Energy Onyx Pulsar AM transmitter. As such, they are more than capable of receiver operation and could likely handle a fair amount of RF power in the transmit mode.

CDM F2B 0.01 uF capacitor with back of N connector inputs

Finding a type F2B 0.01 uF capacitor, rated at 2000 volts and 11 amps, the value of the inductor was calculated. For the inductor, a 20 uH coil with taps will work great. For receive-only applications, much smaller-sized components can be chosen. Also, there are many bandstop filters with multiple poles. Those are great, but I like the simplicity of the parallel resonant LC circuit.

20 uH inductor salvaged from Energy Onyx transmitter

The N connectors were salvaged from I don’t know where and the enclosure used to house a power supply for a Radio Systems console.

For shielding, I sanded the paint off of the enclosure where the lid is attached and tacked some brass screen down with gorilla glue. This will make a good RF contact surface. The outer of the N connectors are bonded to a piece of copper ground strap which also has a grounding lug on it.

Enclosure lid with brass screen to make contact

I used the Libra VNA to tune it up:

The scan shows it is -31 dB on the carrier frequency. It is -17 dB on 760 KHz and -20 dB on 780 KHz. This is good, because I may still want to listen to the station on the remote receiver. According to the smith chart, it is actually resonant on 771.5 KHz, but that is close enough for this application. I think the resonance went up slightly when I put the cover on after the tune-up.

There are several tank circuit calculators online. It is best to have more capacitance and less inductance to keep the Q of the circuit low and suppress the sidebands as well as the carrier.