RF Power Loss Based on ANT 1 or ANT 2 Selection

  • 1
  • Problem
  • Updated 5 years ago
  • Not a Problem
I have a problem (I think) with RF power changes of greater than 15 db depending on the Antenna 1 and Antenna 2 selection. The 4 screen snips below show what I am refering to. Each screen snip was made with no other changes except the antenna changes.
On my 6700 ANT 1 is a Off Center Fed Dipole (OCFD) for 160 thru 6 meters while ANT 2 is a R7 All Band (40 up) Vertical. For the test transmissions seen on the screen snips the ATU was used to tune the antenna each time the transmitting antenna was changed.
The RF Power slider was set to 25 for all 4 tests, SSB mode was used and the tune button used to turn the transmitter on.
With the receiver set to ANT 1 and transmitter set to ANT 1 the RF power peaked to approximatly -25 db as seen on snip 1.
With the receiver set to ANT 2 and transmitter set to ANT 2 the RF power also peaked to approximatly -25 db as seen on snip 2.
However, with the receiver set to ANT 1 and transmitter set to ANT 2 the RF power peaked to only -41 db (approximatly) as seen on snip 3.
Also, with the receiver set to ANT 2 and transmitter set to ANT 1 the RF power peaked to only -47 db (approximatly) as seen on snip 4.
These tests are repeatable and were tried on both 20 and 40 meters. The results don't make any sense to me and I have totally confused myself trying to understand them. It appears to me that if I use the same antenna for both transmit and receive everything is fine. But if I want to use different antennas on the transmitter/receiver setup then somewhere I lose 15 to 20 db.
The reason for me to use different receive/transmit antennas is that on 15 meters the SWR of the OCFD is to much for the ATU to adjust, so I wanted to use the Vertical to transmit with.
I hope someone can help me understand this (maybee) problem by trying it on there 6700.
I'm running Version 1.0.24 on a WIN8.1 HP I7 laptop computer
Al, N4AB
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Posted 5 years ago

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Steve - N5AC, VP Engineering / CTO

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OK here's the deal -- when you are trying to determine your RF output power, you should not look at the panadapter. You should look at the RF Power Out meter in the transmit pane. There is a bridge and power measurement device that will give you accurate results on your output power there. As you can see from each of your screen shots, the power output is the 25W +/- that you stated and this is what is really going out of the radio.

Now, in different antenna output and RF input situations, there different relays and types of relays in-line between the transmitter and the receiver. These relays prevent the TX RF from being sent to the receiver (this would be bad juju of course). From a design perspective, we just make sure that we have at least enough loss to prevent damage to the receiver from the transmitter. The design goal was NOT to provide some sort of accurate RX reading during TX. So you will see the signal at varying levels depending on which type of relay and how many are between you and the transmitter. So the level will vary.

In addition, the spurs you see to the left and right of the antenna are generally not real -- I discuss this in another post here somewhere. They are created inside the radio when there is a lot of high power RF in the chassis. We discussed blanking the display during TX, but all of the engineers in our office preferred to use it as an type of useful diagnostic -- it is in some ways representative since you can see here you are transmitting a single tone. But below a certain level, you have to discount what you see. Ground currents, magnetics and high power RF in the chassis will create some of these spurs in the receiver that you are not transmitting. But we felt most hams would prefer to see what they can see rather than have it "covered up."