HF Transceiver and RTL-SDR Dongle sharing only one antenna, is it safe?

[HoganTyler]

Greetings,

I have an HF transceiver, the TKM707 (150W), which can both broadcast and receive, and an HF antenna on the roof.
I also have an RTL-SDR USB dongle that I wish to use to scan shortwave signals (receiver only).

Can I use both devices with only one antenna by using an antenna splitter? Is there a risk that when I broadcast with the TKM707, the 150W power could go to the USB dongle and damage the PC's USB port? Or the splitter can handle that?

If anyone has faced similar issues, please let me know!
Peace!

 

[K4EET]

Greetings,

I have an HF transceiver, the TKM707 (150W), which can both broadcast and receive, and an HF antenna on the roof.
I also have an RTL-SDR USB dongle that I wish to use to scan shortwave signals (receiver only).

Can I use both devices with only one antenna by using an antenna splitter? Is there a risk that when I broadcast with the TKM707, the 150W power could go to the USB dongle and damage the PC's USB port? Or the splitter can handle that?

If anyone has faced similar issues, please let me know!
Peace!

When you say “antenna splitter”, you are probably referring to a cheap 3 dB CATV type splitter. First those are 75 ohms and not 50 ohms for HF antennas. Second, while that would work for receive ONLY with a mismatch in impedance, you will DESTROY the dongle and probably your computer too if you transmit at any power level. DO NOT USE THAT CONFIGURATION!!!

While there may be a more expensive way to use the single antenna, you would lose the HF bands on your dongle. The BEST thing to do is to use a separate antenna like a discone antenna perhaps for the dongle and mount that antenna a good 30 feet from the HF antenna where you are transmitting with 150 watts.

Let us know if you have any questions.

 

[K4EET]

@HoganTyler, I just noticed that your post was your first and that you are new here. Let me be the first to welcome you to Radio Reference! I hope you find this site very helpful and informative. 73, Dave

 

[bharvey2]

Welcome to the group HoganTyler. With regard to your question, as K4EET points out, the configuration you describe is a BAD idea. It will damage your SDR for sure and might very will harm your computer. While solutions exist to allow radios to share an antenna, those are usually for radios that reside on fixed but separate frequencies. Since it's likely that you'd use both your transceiver and SDR on multiple frequencies over a wide range, it just isn't practical. If you don't intend to use your transceiver and SDR radio at the same time, you could use a coax switch, rated for at least 150 watts and switch between the two depending upon which you are using at the time. If however, you want to use them simultaneously, then you safest bet is to use two antennas as far from each other as practical. If antennas from two radios are too close together, it's possible for the transmitting antenna to induce enough energy into the receiving antenna to harm the receiver. That's the reason behind the antenna separation.

 

[prcguy]

Some HF transceivers have the ability to tap into its internal T/R relay where you can install a two way divider to continue feeding the receiver within the HF transceiver and another receiver. This will allow the SDR to receive simultaneously with the HF transceiver and it will protect the SDR during transmit.

 

[HoganTyler]

Thank you, K4EET and everyone!
That’s exactly what I was thinking when I considered using an antenna splitter. I really thought about using a cheap TV splitter that simply parallels the wiring, without a controller board or concern for impedance (I thought it was similar to an electrical multi-plug and didn’t think impedance would matter).

In my opinion, receiving radio signals is easy, but transmitting is more complex. I once had a bad experience with an Arduino DIY project where 12VDC flowed into my PC and destroyed the motherboard, so I have to be careful from now on.

The HF transceiver I’m using was taken from my family’s fishing boat, which currently operates with an Chinese HF20A antenna.

This antenna has a 100W resistor at the end, which I assume is there to absorb excess energy and prevent it from reflecting back power to the transceiver, and reducing SWR. It should also be a workaround for a full band antenna. I hoped that the resistor could act like a protector, preventing any high power current from reaching the PC via the RTL-SDR dongle. However, it doesn't seem to work that way.

Yeah, one more cheap solution would be using an antenna switcher, which only allows one device to connect to the antenna at a time. But, there’s still a chance I could accidentally transmit when I forget to switch on the antenna.

I’m really grateful for all your advice with real experience.
I think I should make another separate DIY dipole antenna for the RTL-SDR dongle using some spare copper speaker wire, place it a bit away from the broadcast antenna, and nothing to worry about anymore.

 

[spongella]

Welcome to the group! I use a HF vertical ham antenna for both hamming and listening to shortwave. I just connect the coax directly and separately to each radio as needed. Simple and no need for splitter. Have fun on the bands and 73's.

 

[K4EET]

Welcome to the group! I use a HF vertical ham antenna for both hamming and listening to shortwave. I just connect the coax directly and separately to each radio as needed. Simple and no need for splitter. Have fun on the bands and 73's.

Thanks for your input. I think the OP would prefer to be able to use both radios simultaneously, e.g. be able to transmit/receive on HF while also receiving scanner audio at the same time.

 

[HoganTyler]

Some HF transceivers have the ability to tap into its internal T/R relay where you can install a two way divider to continue feeding the receiver within the HF transceiver and another receiver. This will allow the SDR to receive simultaneously with the HF transceiver and it will protect the SDR during transmit.

From what I understand, that relay port will trigger the divider to disconnect the antenna from the SDR while the transceiver is transmitting and reconnect it when the transmission is finished, right?

 

[prcguy]

From what I understand, that relay port will trigger the divider to disconnect the antenna from the SDR while the transceiver is transmitting and reconnect it when the transmission is finished, right?

Yes but probably no. My recommendation is to tap inside the radio to the existing T/R relay that separates the transmitter and receiver, then spilt the signal going to the receiver for your external SDR. The relay port on the back can control an external relay in the antenna path routing antenna to the SDR during receive then switching to the transceiver when you transmit. That means the internal receiver will never see the antenna and will be useless. A better use of the radio's relay jack is to run a relay that will disconnect the SDR speaker or mute the SDR when transmitting as it will probably make some noise or cause feedback when you transmit.

 

[HoganTyler]

@HoganTyler, I just noticed that your post was your first and that you are new here. Let me be the first to welcome you to Radio Reference! I hope you find this site very helpful and informative. 73, Dave

@K4EET
I am planning to install another 4 band antenna (Diamond CR8900) to the roof, for my 50W transceiver TYT TH9800, covering 28/50/144/430 MHz.

I tried to position it as far away as possible from the HF transmission antenna, but the maximum distance is still only 10 feet.
Is there any risk when the HF transceiver is in use? Could it affect the car transceiver in any way, or will it just increase some noise?

I guess the distance might have some relation to the wavelength, for example, 1/4 or 1/2 of the wavelength, similar to power transmission.

 

[bharvey2]

At 10 ft away from each other, I wouldn't be terribly concerned. Consider that folks with both HF and VHF/UHF radios in their vehicles likely have their antennas no more than that distance. My VHF/UHF antenna and HF loop antenna at home are less than 10ft apart and I've never had any issues.

 

[Token]

@K4EET
I am planning to install another 4 band antenna (Diamond CR8900) to the roof, for my 50W transceiver TYT TH9800, covering 28/50/144/430 MHz.

I tried to position it as far away as possible from the HF transmission antenna, but the maximum distance is still only 10 feet.
Is there any risk when the HF transceiver is in use? Could it affect the car transceiver in any way, or will it just increase some noise?

I do not know about the TYT TH9800. But, that radio is a near clone of the Yaesu FT9800R, and I know of several instances of the FT front end getting popped when an HF radio was used in the same vehicle, with the HF antenna 6 to 8 feet away from the VHF antenna. With that said, I have an FT9800R (10/50/144/430) in my truck, and I also have a TS480SAT (all band HF including 50 MHz) in the truck, the antennas are roughly 5 feet apart, and I have had no issues in 4 years of use.

T!

 

[Token]

I think I should make another separate DIY dipole antenna for the RTL-SDR dongle using some spare copper speaker wire, place it a bit away from the broadcast antenna, and nothing to worry about anymore.

Yes, do that. Using a receiver and transceiver (used to transmit, vs just receive) hooked to the same antenna can be possible, with the right equipment and procedures, but it always introduces the potential for something catastrophic to happen.

Even doing what you are describing (separate antennas) there will still be times when there is some danger to the receiver, and so you may want to shut the RX off when the TRX is being actively used. You can run receivers near transceivers, on different antennas, such as many contest stations do, but the RTL-SDR is a very low cost, wide open front end. It is not going to be as forgiving as a high quality receiver would be.

T!

 

[K4EET]

@K4EET
I am planning to install another 4 band antenna (Diamond CR8900) to the roof, for my 50W transceiver TYT TH9800, covering 28/50/144/430 MHz.

I tried to position it as far away as possible from the HF transmission antenna, but the maximum distance is still only 10 feet.
Is there any risk when the HF transceiver is in use? Could it affect the car transceiver in any way, or will it just increase some noise?

I guess the distance might have some relation to the wavelength, for example, 1/4 or 1/2 of the wavelength, similar to power transmission.

It would be possible to calculate the theoretical field strength at the receiving antenna. Then if data were available to show at what RF signal level at the receiver’s front end, after calculating signal loss in the coax run, that it would get “fried”, then you would know for sure if the scenario is safe for the receiver. Unfortunately, that spec for when the front end blows is either not know or not disclosed.

For me, I would be leery of the antenna configuration and perhaps put an attenuator inline with the receiver. Of course, you would be reducing what you’d be receiving if the attenuator were not there. I’ll admit, I’m very cautious…