ACOM-1500 & ANTENNA TUNER?

[VK4CCV]

G'Day Everyone,

Up until now I have been using my FTdx-5000 through an Antenna Tuner then onto the Dipole (which I use on all bands).
No, I have just bought a new ACOM-1500.  ACOM have said to me, as long as the SWR isn't too crazy (below 3:1) then the ACOM will be able to tune the antenna.

My question is: With the ACOM in circuit and turned on to on Standby then the Amp can be used to tune the dipole when needed.   If then Turn Off the amp, what happens then?
Will I need to reintroduce the Antenna Tuner?

Pete VK4CCV

[VK4ADC]

Pete

I am not familiar with the ACORN radio equipment but what the 3:1 SWR rating usually applies to is a measure of the ability to provide maximum output into a matched load, eg 50 ohms.  (Many radios have the same maximum SWR rating - around 3:1.)  If the load Z is between 16 and 150 ohms, then it should produce near the rated maximum power on transmit.

In practice, it is almost always better to run through an ATU (automatic preferred) even though the introduction does cause some in-line power loss.  What they do is synthesise a load impedance close to 50 ohms so the radio can output maximum power regardless of the SWR reflected by the antenna at any given frequency.  They do not improve antenna efficiency, just provide an approximate match.

Me - I'd use the AutoATU all of the time, amp in line or not (within ATU power ratings).  

You can always put the ATU between the Acorn amp and the radio. If the amp isn't on, the ATU matches the antenna Z. If the amp is on, it matches the input Z of the amp. Win-win !

Better still, improve your basic antenna SWR.

[VK5ZD]

When the ACOM is either off or in standby mode the transceiver is connected directly to the antenna.
Go with Doug's suggestion; put the ATU between the transceiver and the ACOM.

[VK4CCV]

Good Morning Gentlemen,

Thanks for the responses.
A bit to consider by the looks of it.

Question: If I was to place the ATU between the Transceiver and the Amp, them when the Amp is switched Off, then I can assume that the relays would switch out which would basically disconnect the Antenna, so having the Tuner between the two would be of no use - unless I am seeing a different picture. :-)

ACOM are normally pretty good with their support, so I will shoot them an email with the scenario and see how they respond.

Thinking out-loud here, but I could use an Antenna Switch???

Pete. VK4CCV

[VK4ADC]

 "could use an Antenna Switch???"  NO, NO, NO.  For a start, you would need TWO. One In and one Out, coax loop between.  Could also cause RF instability.

You will find that virtually any ATU or properly designed power amplifier is a straight-through when set to Off/Standby/de-powered/...

Just put the ATU between the Acorn amp and the transceiver, as suggested earlier.  What damage can it do to try it ?  None.  Reduce the RF drive to the amp if necessary.

Better still, improve your basic antenna SWR.

[VK2CSW]

Would a simple test be to put a VSWR bridge on the output of the radio, connect it to the antenna* and record the vswr.

Then put the amp between the bridge and the antenna and again measure the VSWR in the three states (On, Standby and Off). That should tell you pretty quickly if the amp goes into bypass when it is off.

*substitute a dummy load for the antenna if that is easier.

[VK5ZD]

The circuit (in the manual - available online) shows the input connected to the antenna 1 output when the power is off.