DC voltage on RX antenna

[daschnoz]

I'm setting up a full duplex ammo can repeater with solar power. One of the things that I noticed about others who did this is that they had trouble with the TX radio messing up the solar controller, so they wrapped one, the other, or both in foil. My plan involves using 2 ammo cans. Can #1 will contain the RX and TX radios and a battery. Can #2 will have another battery and solar controller in it. The RX antenna will sit on this can.

 

To keep the connections between the cans to a minimum, I'm planning to send the DC voltage from can #2 to can #1 on the same cable that carries the RX signal to the RX radio. I will use some coils to keep the RF signal out of the batteries, and a cap to keep the DC voltage out of the RX circuit on the RX radio.

 

Do I need to have a cap on the antenna end of the RX line? In other words, is it harmful or detrimental to have the antenna sitting at 12VDC?

[dwmitchell61]

Some antennas may have a DC "short" in them. One such antenna is the collinear antenna as built by WA6SVT. There is a short between the center conductor and shield where the 1/4-wave whip joins the top 1/4-wave segment. That being said, many of the antennas in a fiberglass shell ARE collinear antennas and MAY have this feature. To err on the safe side, I would check your antenna with an ohmmeter before using it in this setup. 

 

The second issue that may arise is any RF noise or ripple that may be sourced at the solar charging controller. I am not familiar with the internals of such controllers, therefore have no knowledge to impart concerning this issue.

[berkinet]

Note that with your design you will be sending both charging current and operating current across the cable. I would suggest you look for a multipurpose cable which actually combines a copper pair (for DC) and a coax. these could be in the same jacket or fused along their length. If you can’t find something you like, then I’d suggest using a small diameter plastic conduit or tube with DC and coax inside it.

 

BTW, since it appears you will not be using a duplexer, how do you plan to get enough isolation between the rx and tx antennae?

[daschnoz]

Note that with your design you will be sending both charging current and operating current across the cable. I would suggest you look for a multipurpose cable which actually combines a copper pair (for DC) and a coax. these could be in the same jacket or fused along their length. If you can’t find something you like, then I’d suggest using a small diameter plastic conduit or tube with DC and coax inside it.

 

BTW, since it appears you will not be using a duplexer, how do you plan to get enough isolation between the rx and tx antennae?

Isolation - Vertical and horizontal offset, which is why I want to keep the connections to a minimum so I only need to run 1 cable. Caps and coils are cheap compared to 50 or more feet of a 2nd cable. I'm not too concerned about having a perfect system. I'm using a BF-888 for the RX radio. A two pack hurts you for a whopping $20, so if I blow up the RX circuit, I'm not going to be too heart broken.

[Lscott]

Isolation - Vertical and horizontal offset, which is why I want to keep the connections to a minimum so I only need to run 1 cable. Caps and coils are cheap compared to 50 or more feet of a 2nd cable. I'm not too concerned about having a perfect system. I'm using a BF-888 for the RX radio. A two pack hurts you for a whopping $20, so if I blow up the RX circuit, I'm not going to be too heart broken.

Do you have a way to check the line impedance with the cap's and RF chokes installed? At UHF it doesn't take much stray capacitance in a coil winding to end up being a parasitic resonate circuit which can totally bugger up what you're trying to do for example.

 

The best way to check the final setup is to substitute a known good 50 ohm RF dummy load, rated for at least 500 MHz, for the antenna on the load end, then check the match on the feed end. The dummy load should have either a SMA, BNC or "N" type connector on it so the connector doesn't affect the measurement. 

 

No electrical component is perfect. Coils have high resistance at RF due to skin effect, stray capacitance between turns. Capacitors also have resistance, dielectric losses and skin effect in the plates, additionally they exhibit inductance due to the finite length of the connections, or in the construction if the capacitor elements are rolled into a cylinder.

 

Then with coax cable you get impedance transformation effects if it's not terminated in the line's characteristic impedance. You can minimize that problem by using multiples of a 1/2 wavelength of cable. Transmission line theory says a 1/2 wavelength of cable will show the same impedance on the feed end as the load end independent of the line's impedance. Remember the wavelength for the cable is the free space wavelength of the signal multiplied by the velocity factor of the cable you're using. For example your typical RG-58 coax has a velocity factor of 0.66 so at 465MHz a 1 wavelength is about 64.3cm in free space and 42.4cm, 16.7 inches, in the coax.        

[Radioguy7268]

This is why FCC Type Acceptance is a GMRS requirement.

 

"Build your own repeater" is a HAM project, not a GMRS project.

[berkinet]

Isolation - Vertical and horizontal offset, which is why I want to keep the connections to a minimum so I only need to run 1 cable. ...

...A two pack hurts you for a whopping $20, so if I blow up the RX circuit, I'm not going to be too heart broken.

It is going to be, at best, difficult getting the separation you want while keeping your cable runs (and now I²R losses) low. So, if you are willing to risk $20 by taking a chance at blowing up your radio, why not just spend a little more and get a cheap UHF duplexer?  That way you can put Tx, Rx & the antenna in one box and the power supply elsewhere, connected with low-cost #12 or #10 wire.  

[daschnoz]

It is going to be, at best, difficult getting the separation you want while keeping your cable runs (and now I²R losses) low. So, if you are willing to risk $20 by taking a chance at blowing up your radio, why not just spend a little more and get a cheap UHF duplexer?  That way you can put Tx, Rx & the antenna in one box and the power supply elsewhere, connected with low-cost #12 or #10 wire.  

OK, change of plans..... (I do this often - over-complicating things only to end up landing on a much more simple solution)

I've had a duplexer on my wish list for a while, and just pulled the trigger on it this morning after reading your previous post.  I will  put  the duplexer in the same box with the solar controller, and run 3 short jumpers between the 2 boxes - RX,TX, and 12V power. 

 

This will be family comms while we are camping where there is no cell signal.  I'm trying to keep this a 2 box project in case I need to hike up the mountain for a good repeater site.  If the camp site is centrally located, the repeater will be there.   I suspect this will seldom be the case though.  I can think of at least 4 parks we plan to visit again where I will be humping this thing into the woods.  I did try the simplex repeater thing, but the kids could not get the hang of waiting for the roger beep and would constantly step on the re-transmitted signal.

[berkinet]

... I will  put  the duplexer in the same box with the solar controller, and run 3 short jumpers between the 2 boxes - RX,TX, and 12V power. 

...

 

Just curious.  Why not put the duplexer in with the radios, even if it means a bigger box.  You could use small size coax like rg214 with no concerns about loss, but increased flexibility.  Then, you could make up sets of 2-conductor power cord with power-pole connectors - and use as many/few as necessary, and locate the radio box way up in a tree if you wanted.  You could also put both batteries in with the charge controller for better use of space.

[daschnoz]

Just curious.  Why not put the duplexer in with the radios, even if it means a bigger box.  You could use small size coax like rg214 with no concerns about loss, but increased flexibility.  Then, you could make up sets of 2-conductor power cord with power-pole connectors - and use as many/few as necessary, and locate the radio box way up in a tree if you wanted.  You could also put both batteries in with the charge controller for better use of space.

I may do that.  I'll figure that out once I get my hands on the duplexer and measure it.  As usual, the spec for this project is evolving as the project moves forward.

[daschnoz]

UPDATE:::

 

I'm planning to use SO239/PL259 connectors for all of the external connections, which means that the metal ammo cans will share the radio ground.  It's a good thing that I checked the grounding on the solar system.  The solar controller is positive ground, so everything between the panel and the battery output will need to be isolated from the radio ground.  That means that I will NOT be able to put the solar controller in the box with the duplexer.  Instead, I will mount the controller on the panel and run the battery line from the controller directly to the can which contains the battery (12aH SLA), so I will no longer need to put DC voltage on the RX cable.  Sure, I could use a TRS (T and R only) or XLR connector, but then I need a dedicated cable type (believe it or not, cabling is the part that I planned to keep simple from the start - it's all using the same connector).