I work in a small music recital hall with 18-inch thick concrete walls. We use Kenwood TK-2400V4P radios which aren't able to cut through the concrete, so I picked up a Kenwood TKR-750 repeater.
I'll see if I can help you out here…
The TK-2400V4P are 4 channel VHF radios.
The P means they are the "ProTalk" line, which are pre-programmed VHF radios. They give you the option of running up to 4 of many various VHF business frequencies.
The TKR-750 is a VHF repeater.
You're on the right track.
Issue is that for a standard repeater like you are looking for to work, you -must- have two separate frequencies. One frequency is the repeater input, the other is the repeater output.
If you try to set up the repeater to work on the same transmit and receive frequencies, you are going to have multiple issues that is going to keep this from working.
Like when you hold a microphone too close to a speaker, you are creating a feedback loop. When you try to run a conventional repeater like this on the same frequency, when it transmits, it's going to key up the receiver. The receiver will key up the transmitter and pass audio. Repeat until something fails.
Since the stock TK-2400V4P's only do simplex (same transmit and receive frequency), you are not going to be able to make them do a separate transmit and receive frequency that is required to make a conventional repeater work.. So, as it stands, you don't have the right radios to make this work the way you want.
I was able to get it programmed to work with our radios
I'd be curious how they programmed it to work with simplex radios. While you can program it to work as a base station radio, to work as a repeater, it would need to have separate transmit and receive frequencies.
There are ways to "trick" the TK-2400V4P's into thinking they are the non-ProTalk radios, and allow them to be programmed with the separate frequencies, but this is a whole different discussion.
I was able to get it programmed to work with our radios, but as I'm new to this area, I had originally purchased 3 cheap VHF antennas and tried daisy-chaining them with the TX and RX jacks on the repeater, with very limited improvement in transmission.
If you connected the transmit and receive jacks together, you very likely damaged the receiver. Feeding the output of the transmitter (high wattage) into the receiver (expecting to see signals in the millivolt range), it's not going to work out well. Think of plugging the output of a large audio amplifier into the mic input of a really nice mixer. Poof, magic smoke gets released.
If you had them separate, it's likely not going to work very well since without some filtering, the transmitter, even on a separate antenna, will still vastly overwhelm the receiver, even if it's on a separate antenna without a -LOT- of separation between the two.
I'm now realizing that I would be better off getting one good antenna. Here's where I need your help.
Not just an antenna. You need a duplexer, which is an RF filter that is tuned to the specific two frequencies you use. One side passes the transmit frequency while blocking the receive frequency. The other side passes the receive frequency while blocking the transmit frequency. If you are going to use this as a conventional repeater, you are going to require that piece of equipment. It cannot be purchased off the shelf and plugged in, it will require tuning to the specific two frequencies.
1. To confirm, I don't need a repeater since I'm using the same frequency for TX and RX, right?
If you want to use a conventional repeater, then you will need two separate frequencies.
If you only want to use one frequency, then you either don't use a repeater, or you use a "simplex" repeater, which basically is an audio recorder that picks up the transmissions and sends them back out on the same frequency when the person transmitting stops. Issue with those is it severely delays the radio traffic, and those within range of both radios will have to hear every. single. transmission. twice. = and those within range of both radios will have to hear every. single. transmission. twice.
2. Should I use a splitter to join the RX and TX jacks into one antenna, or do I need two separate antennas?
To do this well, you need a duplexer tuned to your specific frequencies. A "splitter" isn't going to work unless it is a tuned device with lots of isolation between the ports.
You can use separate antennas, but they need a bunch of vertical and/or horizontal separation to keep the transmitter from deafening the receiver. This will not work well in a small venue, as the distances needed will result in poor coverage.
3. Any recommendations on which antenna should I get? I've been looking at the DB222-A. I need something I can mount indoors. We're also very tight on funds and I will have to make a solid case for whatever I decide on before getting approval to spend the money.
A DB222 would be total overkill. If this is indoors, you can do with a much smaller antenna.
4. Antenna placement... I'm dealing with two 18-inch thick concrete walls. The repeater is on one side of the two walls, where we have some people on radio. We also need to communicate to the people between the two walls, as well as people on the other side of the two walls. Should I place the antenna between the two walls or should it be near the repeater?
This would be a difficult question to answer without seeing the location. Ideally you'd want the repeater antenna to be in a location that can "see" all the areas you want to cover. Some signal is going to bounce around and make it where it needs to be. Some will get through the walls. The repeater, having a better antenna and better receiver than the portable radios is going to work better. It will also (probably) transmit with more power, so will have better coverage.
The only way to get signal to the antenna if it is between the walls is via 70-ohm coax built into the building (for SDI video), and I understand that this could introduce signal loss issues since we're dealing with over 100-feet of cable length, am I correct?
Yes, you are correct. Unless it's high grade cable TV type hard line, you are going to have some significant loss issues. There's also an impedance mismatch between the 50Ω that the radio and antenna are going to want to see and the 70Ω (75Ω?) that the coax will present. Not a show stopper, but not working in your favor, either.
So, the $100.00 question that you didn't talk about is FCC licensing…
Unless you are using these radios on one of the 5 Multi Use Radio (MURS) frequencies, you will need to have an FCC license to transmit.
If you are going to use a repeater, you cannot use MURS, and you'll need to have -both- frequencies licensed.
Getting VHF frequencies in your area isn't going to be easy. They are heavily used. You would need to work with a frequency coordinator to find available frequencies and then apply to the FCC for the license.
Frequency coordinators don't work for free. So doing this on the cheap isn't an option.
I can appreciate your desire to do this yourself. You are wise to seek advice, but looking for advice on a hobby/scanner website isn't the right resource. There's a lot involved, financially, legally, regulation wise, etc. Cheap is possible, but different folks have different ideas about what "cheap" is.
I'm happy to help, but you've got a steep learning curve ahead, and you are going to need to spend more money to make this work correctly.
In reality, you probably should have gone with some higher powered UHF radios. UHF has better building penetration in most cases, and likely would have worked on their own in most cases. The licensing issues remain, though.
