[Mich VHF UHF Society] Transverters vs. native rig support
David Pruett
k8cc at comcast.net
Wed Jun 15 11:27:25 CDT 2016
Patrick,
You pose a very good question. As someone who just became QRV on 1296
for this past contest, I have a stake in the choice.
The fundamental impediment to performance on 1296 is feedline loss.
Even good feedlines start to get pretty lossy on 1296. RG-213 has 10.1dB
of loss per 100' (only 9.8% of the signal makes it to the other end),
9913 is 5.2db/100' (only 30.2%), and LMR-400 is 4.8dB/100' (only
33.1%). Even 1/2" Heliax has 2.7dB/100' of loss at 1296 (only 53.7%).
When using a transceiver with 1.2 GHz capability built in, (or a
transverter in the shack for that matter), feedline loss impacts your
station on both transmit and receive. Your transmit ERP is less, and
your minimum discernable signal (MDS) is also less.
There are ways around this, of course. Some people mount an LNA (low
noise preamp) at the top of the tower, along with relays to switch it
out of line on transmit. With the preamp near the antenna, it sets the
MDS for the system on receive, and its gain compensates for the feedline
loss. But it is all too easy to turn the preamp into a smoldering blob
of galium-arsenide if something goes wrong (relay cable is left
unhooked, relay power supply fails or is left turned off, etc.). A
safer and more robust system configures the T/R relay on the tower to be
powered on receive rather than transmit, and often has a separate
feedline for receive only which can be of lesser quality than the main
feedline (since the LNA gain usually makes up for it). But it also
requires an additional antenna relay in the shack, or a xcvr/xvtr with
separate rx and tx connections.
On-line, K5TRA describes an RF-sensed remote box he built to mount near
the 1296 antenna containing an LNA and a 100W amplifier. This is an
elegant solution requiring only a single feedline. However, it requires
home-brewing and is not a trivial project. Check out
http://www.k5tra.net/TechFiles/1296%20MHz%20Remote%20100W%20PA%20and%20LNA.pdf.
Some stations have gotten around the feedline loss problem by using
atransverter and mounting it near the antenna at the top of the tower in
a weather-proof box. This alleviates the feedline problem by putting
the receiver ahead of the feedline loss. Also, it shifts the frequency
on the feedline down from 1296 MHz to the transverter IF (usually 28 or
144 MHz) where the loss is much less. One possible downside is that the
outdoor temperature extremes could cause the local oscillator in the
transverter to slowly shift from it's nominal value, but this should
have only minor impact on it's operation.
My vote is for a good transverter. At my station, I have Down East
Microwave (DEM) transverters for 222, 902 and 1296 MHz and I have never
been disappointed in their performance. Yes, transverters require some
effort to interface to the IF rig, and band-changing is often (but not
always) more complicated the just hitting the BandUp/Band/Dn buttons on
a xcvr. But IMHO the DEM transverters are at least as good (and often
better) than a xcvr with "native" capability for that band.
So, while either the "native transceiver" or transverter approach can
work, I encourage you to look at the decision from the feedline loss
standpoint and choose the approach which will provide your station with
the best performance.
This is an example why I like this VHF/UHF stuff - sometimes it ain't
easy...
73, Dave/K8CC
On 6/15/2016 7:00 AM, Patrick Thomas wrote:
> I'm not sure if this is a holy war question, a newbie question, or both, but here goes.
>
> As a TS-2000x owner, I am fortunate (I guess? :) to have the ability to just click the band-up button and get on 1.2GHz. I've been considering upgrading to an IC-9100 with its 1.2GHz module.
>
> But as I consider the fact that the 1.2GHz price adders for these units is already $400+, and even conventional HF/VHF/UHF rigs seem to come with higher prices and more tradeoffs, I find myself asking: "Is this really any better than a transverter?"
>
> Or is it perhaps even worse than using a high-quality transverter, since many EME people seem to prefer high quality HF radios paired with DEMI or Kuhne transverters. Or just "different" and they're both about the same until you start developing an interest in using a high-accuracy external frequency source, etc.
>
> Thoughts?
>
> - Patrick, KB8DGC
>
>
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