1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

A bypass circuit switch for a car antenna booster (model hd-rx8) that doesn't have one built in.

Discussion in 'Radio and Communications' started by thebestofall007, Sep 1, 2016.

  1. thebestofall007

    thebestofall007 New Member

    Joined:
    May 7, 2016
    Messages:
    21
    Likes:
    1
    Hello all, I have a MMIC based car antenna booster, model hd-rx8, that like most antenna amplifiers, overloads the tuner front end when in the city where stations are already strong, but works wonders when in the boonies far away from stations (it actually amplifies well without adding noise, as well as not hammering the AM reception). The problem is that this booster does not have a bypass switch at all. Simply disconnecting the power wire while it is connected inline between the antenna and radio is not a solution, as it then has no reception whatsoever, and removing the device and directly connecting the antenna cable to the radio corrects the signal overload problem, but of course I then can't use the booster, as it's not connected.

    Here's the deal: I would like to be able to use a toggle switch that turns off the booster and bypasses it, so that electrically and RF-wise the circuit would be invisible to the signal, where the circuit maintains the proper antenna/cable impedance and whatnot, but when on, connects the booster and routes the signal through it as if were connected in-line. Are there any circuits I can make that achieve this? I want to do this because the booster works when reception is overall weak, but when they are strong, I would need a way of bypassing it, since obviously I can't swap the booster out when driving down the road.
     
    Last edited: Sep 1, 2016
  2. MikeMl

    MikeMl Well-Known Member Most Helpful Member

    Joined:
    Mar 17, 2009
    Messages:
    11,057
    Likes:
    541
    Location:
    AZ 86334
    A double pole relay? What are the in/out connectors like? The old Motorola type that look like this?
     
  3. thebestofall007

    thebestofall007 New Member

    Joined:
    May 7, 2016
    Messages:
    21
    Likes:
    1
    Yes. There is a male connector for the output to the radio and a female connector for the input from the antenna.

    A double pole relay is on the right track, however I tried connecting the center conductors and outer braids on both ends together directly to bypass the booster and that made the reception worse. I was puzzled on why. I remember looking at the circuit board of one of those older rectangular "black box" boosters that mount into the dash with a DPDT bypass switch and it didn't just simply connect the two center conductors of the input and output together. There was a circuit with capacitors for impedance matching or something like that. The switch operated two circuits, the booster itself, and the impedance matching circuit that bypassed the booster circuit. That was a while ago, and I don't have that booster anymore, otherwise I would trace that circuit and figure it out.
     
    Last edited: Sep 2, 2016
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. Les Jones

    Les Jones Well-Known Member

    Joined:
    May 15, 2015
    Messages:
    1,447
    Likes:
    188
    Location:
    Lancashire UK

    I am not clear what you mean in this statement in post #3 "I tried connecting the center conductors and outer braids on both ends together directly to bypass the booster and that made the reception worse." This sounds the same as plugging the antenna straight into the radio. (Or do you mean that you shorted the input and output of the booster together ?) This is how I would bypass the booster using a DPDT relay.
    Relay bypass.jpg
    The booster would be in circuit when the relay is energised as the circuit is draw but could be changed to work the other way round by swapping over the NC and NO contacts. Another solution may be to switch in and out an attenuator on the output or input to the booster amplifier. The choice of inserting the attenuator in the input or output depends on weather the overloading is occurring in the booster or the radio. One problem with this approach is that the impedance is probably unknown so making the attenuator would be trial and error. (Rather than being able to calculate the values.)

    Les.
     
    • Agree Agree x 1
  6. thebestofall007

    thebestofall007 New Member

    Joined:
    May 7, 2016
    Messages:
    21
    Likes:
    1
    The booster was overloading the tuner whenever the vehicle was travelling in a strong signal area, like let's say near the station's towers, because it amplifies an already strong signal to a point the tuner's front end cannot handle it. The booster works awesome when the overall reception is weak. Without the booster, the radio works fine with strong signals (I am running Alpine's MaxTune pro tuner and Pioneer's Supertuner IIID, both great tuners). I merely shorted the center coaxial conductor of the input to the center conductor to the output without removing the booster from the circuit. The DPDT relay looks to be the solution because when I disconnect the booster and plug the antenna plug directly into the radio, I am switching 2 connections, the one on the input and the one on the output. Your picture looks like it might work. I may try that.

    I just wasn't sure if I needed impedance matching or not, based on my observations from when I took apart the other booster a while ago. It quit working because the spring loaded switch broke, and the HD-RX8 I have is a better one. Only one way to find out.
     
    Last edited: Sep 2, 2016
  7. thebestofall007

    thebestofall007 New Member

    Joined:
    May 7, 2016
    Messages:
    21
    Likes:
    1
    I found a pair of single pole double throw relays from the old power antenna relay box on my car that might do the trick. I would use one at the booster's input and one at the output, where the coils are energized at the same time and both switch between the booster and bypass. The relays will be at the inputs and outputs to the bypass circuit when no power is being applied, while they are switched to the booster when power is applied, as well as having power go to the booster. The positive wire of the booster will connect to the power wire to the coils.

    I like this forum. I think I might stay here.
     
    Last edited: Sep 2, 2016
  8. MikeMl

    MikeMl Well-Known Member Most Helpful Member

    Joined:
    Mar 17, 2009
    Messages:
    11,057
    Likes:
    541
    Location:
    AZ 86334
    This is what you should be doing. Note that only the center-conductor is switched..., but the coax shield is continuous.

    rfby.png
     
  9. thebestofall007

    thebestofall007 New Member

    Joined:
    May 7, 2016
    Messages:
    21
    Likes:
    1
    That looks to be what I need except I probably will be using a pair of SPDT relays I found, one at each end of the signal chain like you pictured to do this and make the circuit on a double sided board, one side the circuit side and the other a ground plane. I have a metal project box as well. Would this work?
     
    Last edited: Sep 2, 2016
  10. MikeMl

    MikeMl Well-Known Member Most Helpful Member

    Joined:
    Mar 17, 2009
    Messages:
    11,057
    Likes:
    541
    Location:
    AZ 86334
    Should work. I have used normal (small) relays for antenna switching at 2m (144MHz).
     
  11. thebestofall007

    thebestofall007 New Member

    Joined:
    May 7, 2016
    Messages:
    21
    Likes:
    1
    I just thought of something else: Would there be a way to make a setup that analyses a selected band's stations based on signal strength, like a PLL circuit or something like that, and automatically cuts off power to the booster when any station(s) gets within an upper threshold? You know, make this in addition to the bypass circuit and use as a conditional toggle switch?
     
    Last edited: Sep 6, 2016
  12. MikeMl

    MikeMl Well-Known Member Most Helpful Member

    Joined:
    Mar 17, 2009
    Messages:
    11,057
    Likes:
    541
    Location:
    AZ 86334
    You are building a second receiver. Much better to dig into your existing in-dash receiver and fish out the agc/avc signal (used internally to do the same thing: i.e. reduce the RF gain of the receiver front-end to prevent overload). Based on the level of the agc/avc signal, you could switch out the external amplifier...

    The level at which you switch out the external RF amp would be different on AM vs FM, so you also need to bring out a "band" signal...

    Have you tried a better receiver without the external preamp? OEM in-dash radios (believe it or not) are usually better quality receivers (from a weak-signal RF and overloading standpoint) than most aftermarket stuff...
     
    Last edited: Sep 6, 2016
  13. thebestofall007

    thebestofall007 New Member

    Joined:
    May 7, 2016
    Messages:
    21
    Likes:
    1
    This was just a thought. I wouldn't know where to start when it comes to tearing into a tuner. If I had an old OEM radio I didn't care about, I'd play around with it. My vehicle didn't have the OEM deck though, as the previous owner had it quit on him and he took it out.

    I have, however noticed the newer OEM decks have better weak-signal RF and overload performance like you mentioned, having been pleasantly surprised by the performance of the stock radio in a 2015 Dodge Ram 3500 pickup. My vehicle is a lot older than this, and the OEM isn't nearly this good.

    The Alpine deck I have now doesn't overload easily and does a great job rejecting overload unless I leave the booster on when I drive by the towers (and that issue has happened to every radio I've had it connected to). I have noticed that Pioneer's supertuner IIIDs have well above average sensitivity and selectivity, but it's main shortfall is that they de-sense/de-tune BADLY when in a strong signal area and the effect is quite annoying (booster or no booster). You know when the tower you are driving by is a radio station when the tuner's audio gets quiet and distorted on any station that isn't local, moderate strength and weaker stations flutter in and out and start picket fencing badly, and the tuner acts like the antenna's mast come undone from the fender mount sensitivity-wise. The Alpine I have now doesn't do any of that and it performs well.
     
    Last edited: Sep 7, 2016
  14. thebestofall007

    thebestofall007 New Member

    Joined:
    May 7, 2016
    Messages:
    21
    Likes:
    1
    Update: I tried the circuit in post #7 and while it works, I fried my booster in the process because of the back-EMF from the relay's coils and I have to order another one. I think the relays need a set of 1N4007 or such flyback diodes that protect the booster from the voltage spike that comes after the collapse of the magnetic fields in the coils. I felt a nasty jolt when I disconnected the circuit as the relays clicked off and I think that spike killed my booster, which is transistor based. I want to have power to the coils and the booster at the same time when I use a toggle switch. You may want to update this circuit in the diagram, but as for me: lessons learned.
     
    Last edited: Sep 23, 2016
  15. thebestofall007

    thebestofall007 New Member

    Joined:
    May 7, 2016
    Messages:
    21
    Likes:
    1
    Another update:

    Well, I just got through doing this circuit again, and now it works great. You do indeed need those flyback diodes in parallel to the coils in the relay(s), with the mark (cathode) connected to the coil's positive and the positive (anode) unmarked end connected to the coil's ground, as the inductive spike WILL fry any electronics connected to that circuit, especially any transistorized components. Here is an image of what I am talking about, for anyone who is interested in using this circuit: [​IMG]
    The load in this case is a transistor, and the flyback diode protects it from being spiked by the back emf coming off the relay's coil when power is removed.
     
    Last edited: Dec 4, 2016

Share This Page