[amsat-bb] Elk Antenna, Midland CT 890 and Full Duplex

Dani EA4GPZ daniel at destevez.net
Wed Dec 16 17:21:12 UTC 2015

El 16/12/15 a las 16:59, Felix Paez escribió:

> The problem comes when I connect the Elk antenna to it. If I press the PTT
> the receiver noise goes to maximum. I tried to make a contact with the
> SO-50 but I could not hear anything when I pressed the PTT. All was noise.
> I had to disable the Full Duplex and then I was able to make some nice
> contacts (EA1JM, CU2ZG…) in half duplex.

Hi Felix,

Probably you are experiencing desensing on the receiver caused by the
transmit signal. This is much worse when working V/U because the 3rd
harmonic of the TX signal lies on the 70cm band. Patrick WD9EWK has made
many tests with different inexpensive Chinese full-duplex radios and it
seems that most of them are fine to work AO-85 (which is U/V) but it's
impossible to work SO-50 with any of them. Perhaps you could have more
luck with AO-85.

> So my questions are…
>  1.   Is it possible to use a single Log Periodic antenna (only one feed
> point) as the Elk to work Full Duplex or two antennas are needed?

Yes. It's technically possible, although, as I said, desense can be an
issue. I think that it's worse with the elk than with, say the arrow
(which is two crossed yagis) because the coupling between the TX and RX
is much greater on the elk than on the arrow.

> 2.     In case that it is possible… Would this improve by using separate
> receiver and transmitter with a filter that gives a feed point for VHF and
> another one for UHF? (the Elk antenna only has a feed point)

If you have a 2m/70cm duplexer, you can try using two radios. Perhaps a
cheap Baofeng for TX and the Midland (which is more sensitive) for RX.
This will probably be much better than just using the Midland
full-duplex, because the external duplexer will provide better isolation
than the one on the Midland.

> I have to make more tests but at first time, It does not seem possible to
> use the Midland CT 890 (and maybe other full duplex handys?) with the Elk
> antenna in Full Duplex…

It seems that it's difficult to get a handheld that can do V/U
full-duplex. However, U/V is much easier.



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