FT857D antenna ports - a weird one.

[VK2GOM]

I've discovered something quite strange...

Today, I had two VHF/UHF antennas conencted to the VHF/UHF port on the rig via a Comet two way switch. One antenna was a 2m/70cm dual band, the other a 6m/2m/70cm tri brand.

To the HF port on the rig, I had connected my Terlin Outbacker via an LDG auto ATU, with the antenna set to the 12m tap for an experiment to see if it would load up on 6m.

I got it to load up, to some extent. Whilst listening to a weak beacon, for some reason I flipped the antenna switch for the two other (unrelated) antennas onto the 6m/2m/70cm antenna and the signal came up about 2 S-points.

The manual says the ports are HF/6m, and VHF/UHF on the other one.

Intrigued, I unplugged the HF antenna entirely, and still had a good signal on 6m. I then did the reverse, leaving the HF antenna plugged in, and took the 6m/2m/70cm antenna out of circuit, and the signal went back down 2 S-points.

So what's going on? Why is an antenna on an unused (supposedly switched out by the rig) antenna port 'helping' the situation? I'm entirely confused

Is this just a quirk in that the VHF/UHF port doesn't expect to see a 6m antenna on there?

73 - Rob VK2GOM /G0MOH

[VK4TI]

Hi Rob,

I will go and check mine and see whats the go..I seem to recall something similar last weekend in the VHF Field Day but after being curious for five seconds opened another beer..

They are great rigs and certainly help me get a spark of enthusiasm for VHF UHF again - why I even found my old 2 and 70cm transvertors yesterday hihi..

Will email direct OK - need to find a copy of the circuit...

Cheers

VK4TI

[VK2GOM]

Hi Trent,

I'm pretty sure the isolation between ports is via a relay, as I hear all manner of things 'click' as I change through the bands. The thing that got me, even with no HF antenna plugged into the HF/6m port, I was STILL revceiving the 6m beacon. This suggests the ports aren't isolated at all.

73 - Rob VK2GOM / G0MOH

[VK7PD]

Being a proud FT857D owner, I'm interested in this phenomenon myself. If anyone comes up with an on-line schematic of the rig, would you mind sending me the URL?
73,
Peter

[VK2GOM]

Hi Peter,

I have the actual schematic as a .pdf in the service manual - I could email it you? PM me your email address and I will send it on.

73 - Rob VK2GOM / G0MOH

[VK2TDN]

Hi Trent,
I'm pretty sure the isolation between ports is via a relay, as I hear all manner of things 'click' as I change through the bands. The thing that got me, even with no HF antenna plugged into the HF/6m port, I was STILL revceiving the 6m beacon. This suggests the ports aren't isolated at all.
73 - Rob VK2GOM / G0MOH

hey Rob,

others overseas have also noted similar weird things like you have said have seen comments in the FT897D yahoo group

I cant understand why it should happen when there is absolutely no connection between the HF/6M and
the V/UHF ports
The HF/6M ports are all relay switched to get the appropriate band filters before going to the antenna socket

The V/UHF goes from the output transistor via pin diode switching and band low pass filters to its own antenna
socket.... and Never the twain shall meet !!! There is NO relay switching in the V/UHF TX/RX circuitry

cheers
Dave
VK2TDN

[VK2TDN]

Rob and Co,

the only thing that I can think of that is happening is that the bleedover/mixing ... whatever you want to call... it is
happening at the lower common IF freqs. Its the only place in the where the signals get split before going to the
appropriate output sections

Dave
VK2TDN

[VK2GOM]

Hi Dave,

Interesting...

If the ports aren't truly isolated in Rx, what about Tx I wonder? Perhaps transmitting 100W on 6m could destroy a transverter connected to the VHF/UHF port even?

73 - Rob VK2GOM /G0MOH

[VK6ADF]

Hi Dave,

Interesting...

If the ports aren't truly isolated in Rx, what about Tx I wonder? Perhaps transmitting 100W on 6m could destroy a transverter connected to the VHF/UHF port even?

73 - Rob VK2GOM /G0MOH

I have seen this warning on advertisements for a particular autotuner.

Not for use with FT-857/897 radios if you plan to use V/U because these radios transmit a small amount of V/U RF energy from their HF port and will fool the tuner into tuning.

So could be the other way round as well.

73 Phil...VK6ADF

[VK2TDN]

Hi Dave,
Interesting...
If the ports aren't truly isolated in Rx, what about Tx I wonder? Perhaps transmitting 100W on 6m could destroy a transverter connected to the VHF/UHF port even?
73 - Rob VK2GOM /G0MOH

actually thats NOT the worrysome problem if you wanna use a transverter.... the real prob is and not only with the
Yaesu 857 and 897 's (if my recall is correct the Icom IC706 range also) is that they produce a substantial brief
power spike on transmit ie ... even if you have wound the power down to the 5W minimum (say on 144MHz) to feed
a xverter there is still a very brief almost full power (~ 40-50W) spike before the set 5W is established.

Now I personally have never heard anyone damaging a xverter because of the spike, it is something to be aware of.

If the ports aren't truly isolated in Rx, what about Tx I wonder?

they are definately totally separate (isolated) from the final TX/RX stages to the antenna ports ... a quick look at the
cct diag's will show that. But they do use common freq generation stages and of course mic amp etc

Dave