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eHam Forums => Elmers => Topic started by: IZ2HDN on February 12, 2010, 02:18:54 AM



Title: send VCC thru coax
Post by: IZ2HDN on February 12, 2010, 02:18:54 AM
i realize this could be a quite silly question..my IC910H sends Vcc thru coax to a remote preamplifier.Once on the mast,how can i get Vcc out from remote cable to feed into a UHF preamplifier which has a Vcc supply separated from RF?
Thanks!
'73 from IZ2 HDN Max


Title: RE: send VCC thru coax
Post by: W5FYI on February 12, 2010, 03:09:05 AM
The usual scheme is to tap voltage off the center conductor through a radio-frequency choke (RFC), and to let the RF from the preamplifier pass into the center conductor through a feed-through capacitor. I am guessing that is the method used by your radio.


Title: RE: send VCC thru coax
Post by: KG4RUL on February 12, 2010, 04:46:56 AM
One of these at each end and you are golden.

http://www.mfjenterprises.com/Product.php?productid=MFJ-4116


Title: RE: send VCC thru coax
Post by: IZ2HDN on February 12, 2010, 05:08:27 AM
Many thanks Dennis.Good device.
I probably forgot to tell that i am injecting UHF RF into coax.The MFJ device has a cut off frequency of 60 MHZ and i am sending out 430 MHZ.
Best '73
IZ2 HDN Max


Title: RE: send VCC thru coax
Post by: KZ1X on February 12, 2010, 07:02:51 AM
You can build the same basic 'bias-T' circuit with UHF techniques and component values.  Commercially available versions exist, of course, but at a price.


Title: RE: send VCC thru coax
Post by: N3OX on February 12, 2010, 07:12:35 AM
I think you can use an open circuit 1/4 wave stub and a short circuit 1/4 wave stub to make a single band bias tee.

You use the open circuited stub, which is a short circuit at the desired RF frequency and an open circuit at DC, to bring the RF off the coax, and you use the short circuited stub, a high impedance at RF and a short circuit at DC, to pass the DC off the coax.

You can do it with lumped elements too, but I think maybe for one band on UHF, a transmission line version might be more straightforward.

I haven't tried this.   I have built bias tees with chokes and capacitors for HF and am using a couple to send DC control voltages to a three band relay switched antenna... but the concept is pretty straightforward... a DC blocking element to couple the RF on and off the common point and a DC short but high impedance RF element to couple the DC.

73
Dan


Title: RE: send VCC thru coax
Post by: N3OX on February 12, 2010, 07:50:06 AM
I wanted to find a drawing of what I was talking about, but couldn't find a decent one, so I made one:

http://n3ox.net/files/single_band_bias_tee.jpg



Title: RE: send VCC thru coax
Post by: VK1OD on February 12, 2010, 11:54:29 AM
i realize this could be a quite silly question..my IC910H sends Vcc thru coax to a remote preamplifier.Once on the mast,how can i get Vcc out from remote cable to feed into a UHF preamplifier which has a Vcc supply separated from RF?

Max,

The biggest challenge in making a separate wideband bias tee is to have low insertion loss over a wide frequency range, and apart from the physical construction, making an RF choke that is effective over a wide band is the key. Usually that is done by having two or three chokes in series, each working over a more limited frequency range.

In your case, you may be able to integrate the bias tee in the preamp. Since it is then a single band device, the RF choke becomes somewhat simpler, and a single choke wound on a ferrite bead should be easy. Don't overdo the choke, too many turns brings self resonance down, and the choke is less effective than you might think.

The capacitor needs a generous rating to cope with spikes from relays if they are not quenched effectively.

You will find circuits in lots of preamp circuits. If you build the bias tee into the preamp, a good idea is to use a pair of diodes to isolate the coax fed DC and an external DC feed terminal on the box (which can be handy).

Commercial bias tees for UHF can be very expensive, hard to justify considering the parts cost.

Owen