can you use 2 amplifiers on co-phased antennas?

[crusader]

is it possible,practical or feasable to use seperate power amplifiers on each line in a co-phased array?got a feeling it would be very hard to balance this type of system,has anyone ever tried this?
would be interested to hear any comments from you antenna freaks.

 

[KingCobra_CDX882]

possible........yes
practical......no
feasable.......not really..

co phasing antennas can be a pain..
now doing so with a amp for each even harder..

plus will need use a higher power to power both amps..
further stressing your original radio..

but it can be done..

Later

 

[W5LZ]

If you want twice the power get an amplifier that has twice the output. Wouldn't believe the trouble you'd save.
- 'Doc

 

[2M Guy]

But still victim to the fact of dimished returns. Double the power, you don't double the signal. One half of an S unit, hardly noticeable. Get to a kw, you need three kw to gain one S unit. First few hundred watts are the ones you'll get the most notice from.

 

[Marconi]

2m guy is right, plus your co-phase harness is likely working under a mismatched condition and it is not very likely that a CB amp in that portion of the line system is going to like the input.

 

[W5LZ]

2 M Guy,
Oh, I know, but I was looking for a 'short' way of saying it. Actually, you need 4 time the power, but who'z counting?
- 'Doc


Marconi,
Think about it for a second. The only place you need worry about phasing is between the inputs of the amplifiers, if the feed lines from the amplifiers to the antennas are the same/identical, right? So while you still have twice the problems (actually, since 'trouble' follows the 'square' law, 4 times the problems, if you're lucky) but maybe not 'where' you think it'll be. Of course all that assumes that the amplifiers are identical, are operating in phase, at the same power levels... wait a minute, maybe that's the 'cubed' law instead of 'square'. You think?
- 'Doc

 

[crusader]

thanks all for the replies,i was curious if anyone ran such a system and whether it was feasable.personally i think lifes too short to mess about trying to match up and balance such an array, when you could get good results going in another direction.

 

[Marconi]

Yep 'Doc you are probably right, that was sort of my point. Have you ever tried to put 25 ohms or less of real resistance along with very strong ground losses into a simple CB amp. The whole feed line system in a co-phase harness is what gives the Tx'r the breathing room to work under such a mismatch and you won't find that condition good anywhere on the co-phase harness I don't believe. It is the combination of the two tune feeders coming together that produces anywhere near a match and that also includes heavy ground losses as a result.

Maybe just barely a compromised gain of insignificant results if set up properly, and that is the challenge.

I was around in the 70's and I never heard of co-phasing mobiles until maybe the very late 70's or the early 80's at best. In fact in those days you could not even find 75 or 100 ohm coax unless you went to military surplus, but of course they still use RG/8x mistakenly even today and you are for sure operating with heavy ground losses if you get the match anywhere near 50 ohms at the transmitter.

 

[North Texas Mudduck]

had a drivre come thru town talking about his co phased texas stars
well come to find out he ran a cophase out of his radio out to the 2 texas star 400 then out of each 400 the antenna coax went to each antenna is was a wild looking setup
for some reason somebody was filling the guy a line of smoke tellin him the siganl would be stronger
i asked him how it would be stronger
but he kept running the same setup a long time with no problems
now whether it improves anything i cant see it or hear it
but difinatly a nice looking setup

 

[DTB Radio]

had a drivre come thru town talking about his co phased texas stars
well come to find out he ran a cophase out of his radio out to the 2 texas star 400 then out of each 400 the antenna coax went to each antenna is was a wild looking setup
for some reason somebody was filling the guy a line of smoke tellin him the siganl would be stronger
i asked him how it would be stronger
but he kept running the same setup a long time with no problems
now whether it improves anything i cant see it or hear it
but difinatly a nice looking setup

If you wanted to run cophased amps, that would be the way to do it. Run a pair of RG59 feedlines of exactly the same length from your radio to your amps, then run standard 50-ohm from the amps to the antennas. Using 75-ohm as the jumper between the amps and radio will keep the match between the amps and radio to under a 2:1, probably close to 1.3. Using 50-ohm coax from the amps to teh antennas will keep a nice match between those items. As for phasing problems, I don't think you'd have any real problem there. Assuming you're running to identical amps (a pair of Texas Star DX350's for example), they'll be close enough in phase to negate any phase problem. The rest of the system will negate phase problems if you measure your feedlines correctly, and match each antenna seperately before putting the whole system together.

 

[freecell]

the phasing harness performs the same function regardless whether it is terminated into a pair of antennas or a pair of amplifier inputs. if we have a transmitter output impedance of 50 ohms and two antennas or amplifier inputs representing a pair of 50 ohm loads in parallel or 25 ohms total then the matching line between the two sets of impedances required for matching is *35.35 ohms for a single matching line or transformer device. (in the case of a single 25 ohm load input) since we have two loads presenting a total parallel input impedance of 25 ohms we use two 75 ohm lines in parallel which equal *37.5 ohms, roughly the equivalent of a *1.06:1 swr.

i have installed several of these and i can tell you that the only "problem" that i have ever encountered is getting both amplifiers to key and release in unison. this problem is easily solved by eliminating the rf keying circuits and placing both amplifiers under the direct control of the transmitters PTT control line.

i'm not going to argue the advantages or disadvantages of this particular arrangement but i will say this, using the TSDX350 as the test case. this amplifier with the associated transistor devices (2X2SC2879's) produces a clean 200W PEP with regard to mil spec for IMD3 products and spurious. many will argue that it will do more but the bottom line is that it will not do twice this figure while maintaining any spectral purity of emissions so i am speakiing to those who would be entertaining the use of this configuration for communications purposes and not keydown activities. yes, it's more than capable of being accomplished and depending on your understanding of the acceptable operating parameters of the devices in the amplifiers used, definitely practical. in combination with antennas producing gain, vehicle mounting geometry (antennas) with a view to directionality and the use of dual amplifiers the results are (or can be) more than satisfying. unfortunately the project is beyond the scope of the casual hobbyist and requires a great deal of planning and preparation to produce any meaningful results. furthermore the fun doesn't stop here as even though the spacing between the antennas is fixed, the introduction of leading or lagging phase angles in the lines between the amps and the antennas (whether done manually, with switching arrangements or with the use of phasing circuitry) will also yield a variety of other advantageous radiation patterns to provide even greater operating flexibility. at 400W PEP total from a pair of 350's it is not difficult to achieve ERP levels approaching 2KW in the favored direction of the array/system, particularly when compared to conventional single amp/single antenna systems, and at that without any undue or excessive equipment heat or spurious emission levels.

http://forum.worldwidedx.com/viewtopic.php?p=76817#76817

 

[Toll_Free]

the phasing harness performs the same function regardless whether it is terminated into a pair of antennas or a pair of amplifier inputs. if we have a transmitter output impedance of 50 ohms and two antennas or amplifier inputs representing a pair of 50 ohm loads in parallel or 25 ohms total then the matching line between the two sets of impedances required for matching is *35.35 ohms for a single matching line or transformer device. (in the case of a single 25 ohm load input) since we have two loads presenting a total parallel input impedance of 25 ohms we use two 75 ohm lines in parallel which equal *37.5 ohms, roughly the equivalent of a *1.06:1 swr.

i have installed several of these and i can tell you that the only "problem" that i have ever encountered is getting both amplifiers to key and release in unison. this problem is easily solved by eliminating the rf keying circuits and placing both amplifiers under the direct control of the transmitters PTT control line.

i'm not going to argue the advantages or disadvantages of this particular arrangement but i will say this, using the TSDX350 as the test case. this amplifier with the associated transistor devices (2X2SC2879's) produces a clean 200W PEP with regard to mil spec for IMD3 products and spurious. many will argue that it will do more but the bottom line is that it will not do twice this figure while maintaining any spectral purity of emissions so i am speakiing to those who would be entertaining the use of this configuration for communications purposes and not keydown activities. yes, it's more than capable of being accomplished and depending on your understanding of the acceptable operating parameters of the devices in the amplifiers used, definitely practical. in combination with antennas producing gain, vehicle mounting geometry (antennas) with a view to directionality and the use of dual amplifiers the results are (or can be) more than satisfying. unfortunately the project is beyond the scope of the casual hobbyist and requires a great deal of planning and preparation to produce any meaningful results. furthermore the fun doesn't stop here as even though the spacing between the antennas is fixed, the introduction of leading or lagging phase angles in the lines between the amps and the antennas (whether done manually, with switching arrangements or with the use of phasing circuitry) will also yield a variety of other advantageous radiation patterns to provide even greater operating flexibility. at 400W PEP total from a pair of 350's it is not difficult to achieve ERP levels approaching 2KW in the favored direction of the array/system, particularly when compared to conventional single amp/single antenna systems, and at that without any undue or excessive equipment heat or spurious emission levels.

http://forum.worldwidedx.com/viewtopic.php?p=76817#76817

Mathmatically this all works, however:

What happens when you take into account the reactance on the input of the amps. No transistor amp mass marketed to CBers has much if any input tuning.

I've done the co-phased amp thing in the past, using two device amps to make a 4 device. The people saying it won't work have no idea how RF works... A combiner is nothing more than an impedance transformer, which is ALL a cophase harness is. So, yes, co-phasing amplifiers will work, a co-phase harness is NOTHING more than an impedance xformer.... Same as a combiner. Learn terminology and read books, and it's amazing what this CB stuff is and how it does actually relate to the real world (Ill never forget a major amp builder in the late 90s telling me that CB stuff doesn't always follow textbook stuff... After all, toshiba says their transistors will do 100 watts, what do they know!!!)..

You seemed well versed, so I wanted to pose that question to ya: I've done it, but never bothered to look at the input swr, I just tuned for max suds on the output meter while tuning the tram's tune and load..... Tubes, what SWR?


--Toll_Free

 

[freecell]

"but never bothered to look at the input swr,...."

after looking at it a few times there wasn't enough deviation in terms of R = 50 or X = 0 to worry about it, especially when you consider that if you nail it down at one frequency it's going to shift anyway as the operating frequency is varied. (nothing different here than if we were feeding antennas directly) hey, they're broadband amplifiers. input impedance presents no more of a problem in this configuration than it does in one consisting of a single 50 ohm line and amplifier as long as the "combiner" is properly constructed, all other things being equal.

as i mentioned earlier the only problem i have ever encountered is the synchronous timing of the rf keying circuits. easier to control them from the PTT line than it is to attempt to smooth out any timing differences between the two amp input lines resulting from slight differences in phase angle delay caused by small variations (literally fractions of an inch) in line length or input impedances.