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Reviews Categories | Antenna Switching, Phasing, Controllers | MFJ-1708 RF Sense Transmit/Receive Switch Help

Reviews Summary for MFJ-1708 RF Sense Transmit/Receive Switch
Reviews: 19 Average rating: 3.3/5 MSRP: $79.00
Description: Automatically switches antenna between transmitter and
receiver when keyed.
Product is in production.
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AB9PM Rating: 0/5 Oct 17, 2019 02:29 Send this review to a friend
Junk  Time owned: 0 to 3 months
I got talked into purchasing the SDR duo to run concurrent with my FT DX 1200. I had purchased a MFJ 1708b so I could transmit with my radio, and receive with my computer.

Yes the SDR duo worked with a little encouragement, and when I went to transmit, I transmitted right into my computer. The relays in the MFJ unit didn't fully switch. I had everything wired correctly including the cable from the radio (linear ground connection) so it didn't have to rely on rf sensing alone, as the salesman instructed me to do.

Well I bought it from HRO during their superfest. I was told I didn't wire it correctly. I told them I know the most their salesmen could achieve was the Technician class license, but I actually knew what was going on. Since they sold it as a pair, they would get it back as pair. I was told I would get a restocking fee. A week later I got a call and was told I would get my full refund.

I have made two purchases since, not MFJ and not through HRO.

Beware product may not work as advertised.
K9AQ Rating: 5/5 Sep 16, 2019 13:46 Send this review to a friend
The 2nd one works great  Time owned: 0 to 3 months
I bought an MFJ1708B SDR to replace a homebrew unit that I have been using for many years. I purchased it on line from HRO. There service was great and I got the unit two days later. I hooked it up and no signal to the SDR. I took it apart to look for obvious connection problems but it looks like it was a case of a bad solder connection on the magnet wire used on the transformer. I drove it back to HRO and they gave me a new unit with no questions asked. I brought a wall wart and a VTVM along and tested the replacement unit before I left HRO. The 2nd unit works great and I would recommend it to anyone who wants to had an SDR panadapter to a XCVR.

VE3GLA Rating: 0/5 Aug 27, 2019 03:33 Send this review to a friend
Multiple serious problems  Time owned: 0 to 3 months
I normally donít write negative reviews but I had so many problems from the moment I unboxed it, I had to let people know. This review is for the MFJ1708B-SDR version.
When I took it out of the box, I noticed a loose screw. It was a screw that holds one of the coax connectors to the front panel. I had to take the panel off to re-install the screw using a loose nut that was inside the case. Then I noticed that the two RCA connectors had loose retaining nuts which I tightened.
I then hooked up the device to my transceiver, SDR radio and antenna, and found there was no received signal on my transceiver. I double checked my connection and verified they were correct. I disconnected the device, and used an ohmmeter to see what was going on and verified that there was no connection from the antenna to the transceiver when the mfj device was in receive mode. I then took it out of its case and after a lot of continuity checking, found that the problem was due to a bad connection on the splitter transformer. After a bit of careful soldering I seem to have fixed it.

These were far too many problems for a brand new purchase. MFJ inspector PS-69 initialed the inspection sticker inside the unit. That person needs some serious retraining. Whoever built the device that he or she inspected also needs some serious retraining.
ON5HB Rating: 2/5 Jul 25, 2019 07:46 Send this review to a friend
Not that good, much losses  Time owned: 3 to 6 months
They have done some silly things, JP3 is missing but if you solder a wire over it the losses are far less.
It's stupid there is no jumper placed there as it shorts a splitting-circuit, but that circuit has losses.
Why isn't there a 1KW version? No I have to use it before the Tuner and Amplifier, as such it limits the usable-bands.
However, JP3 missing is a major mistake.
AG4F Rating: 0/5 Jul 6, 2019 19:27 Send this review to a friend
Absolute GARBAGE - even by MFJ Standards  Time owned: 0 to 3 months
I tried 3 of these...3! The first two did not function at all. Power on and both the power and Xmit lights stayed lit full time. No switching. The third one powered up correctly. However, the signal drop when inline is WAY MORE than advertised. So I tried to move the jumper to disable the RF sense, which is supposed to reduce the loss. Unfortunately, it wouldn't switch at all in that position. Yet another piece of mighty fine junk. Why do I even try?
AC6BW Rating: 5/5 Mar 10, 2019 23:28 Send this review to a friend
Works great so far  Time owned: 0 to 3 months
I'm using the MFJ-1708B-SDRS, which is the latest 4 relay, RF sensing version of the 1708-SDR.

I couldn't find it in stock anywhere, so I ordered directly from MFJ. They put the order om backorder. I started looking around, and found one unit in stock at HRO in Minneapolis. I pounced on that one, then cancelled the MFJ order, and got quick delivery from HRO.

QST recently reviewed this unit in the March 2019 issue of QST. I generally agree with that review, and that is what prompted me to buy it. The B version adds additional isolation to the SDR port during TX. It uses 4 relays, and 50 Ohm transmission lines, to achieve the better performance. They recommend using the manual TX control from your rig, instead of the RF sensing circuit, due to a noticeable power spike during the RX to TX transition. I decided to heed this advice, and I have the CTRL input connected to the TX GND output of my rig. To enable this, you do have to open the unit up, and move one of the jumpers, since the default setting is RF Sense. The construction looked good, and I didn't notice any apparent defects.

I am using this unit with a SDRplay RSP1A SDR, and my rig is a FT-450D. I run the SDRuno SW. It's fantastic to finally have a waterfall and spectrum view with the FT-450D! I now can monitor activity on other bands, as well as easily monitor pileup activity, band conditions, noise and RFI, etc.

I haven't had any RFI problem with the unit, considering it has a plastic case. My vertical dipole is literally on the other side of my wall, 15 ft up on a pushup mast, running 600W, and I haven't had any issues. Likewise, I have a hex beam on a 35 ft mast, about 20 ft away from my operating position, and still no problems. I do have a good station ground, with all equipment cases bonded together to an outside ground rod, so that probably helps. I also have good common mode choke filtering on all my coax runs, especially for the vertical dipole, to keep RF off the coax shields. If you are having RFI problems, then I definitely suggest checking all those possible sources.

I tried using the CAT connection feature on SDRuno, to allow me to connect my FT-450D, HRD logging SW, and SDRuno together. This requires OmniRig. This works pretty well, although I have to connect to my FT-450D as a Kenwood TS-480S, and this limits the available controls in my HRD rig control module. I operate a fair amount of FT8, and once I put WSJT-X into the mix, I noticed some instability. Not all of the apps would change frequency in tandem with the others. I operate with CAT disabled most of the time, now, and I'm fine with that.

All in all, I'm very happy with the MFJ-1708B-SDRS, and am thrilled to finally have a panadapter display!
BURGERLOVER65 Rating: 4/5 Mar 6, 2019 18:36 Send this review to a friend
MFJ-1708sdr is worth it  Time owned: 0 to 3 months
So far so good with the MFJ-1708sdr. The best thing about it is I can use seperate antennas for tx and rx by not plugging the tx cable into the unit. The PTT switch is still connected so the sdr radio is still disconnected on tx. I haven't messed with the cat controls since it is ten times easier to change frequencies twice on tx and rx instead of messing with computers. All the cat and Omni Rig mumbo jumbo is enough frustration to shorten a person's life. I learned my lesson long ago.
WB8YQJ Rating: 5/5 Mar 6, 2019 05:04 Send this review to a friend
Good Value   Time owned: more than 12 months
I have the original MFJ-1708. It is a good little HF antenna switch but it does not share the receive antenna so if you want to use your SDR with an HF transceiver for transmit, you have to be listening on the SDR only. The transceiver does not have the antenna until transmit is happening. I like it because I listen to the SDR and transmit on a smaller less expensive FT450D. One downside is the MFJ-1708 AUX relay output used to switch linear amps or mute external receivers has normally closed contacts. I am in the process of installing an additional relay to correct for that. This means there will be two relays clamped while operating the original model, but the MFJ-1708 "B" actually uses four.

I just bought the newer model, MFJ-1708B-SDR. This was designed for use at even greater than 30Mhz - claimed to be good up to 70cm. It does share the receive antenna so I can listen on SDR and FT450D at the same moment... but... there is a small insertion loss, about 3 db that the original model does not have. A new circuit was added to allow proper sharing of the receive antenna, there is a small transformer for that. The original MFJ-1708 can be modified (add a wire) for shared receive antenna but unhappy things (attenuation and crosstalk) can happen depending on the physical devices being shared.

For those with open relay amps, Winkeyer USB has a parameter used to key the amp a specified amount of time before sending the first CW element. Connect the PTT output of the keyer to the control input of the MFJ-1708. On SSB timing on transmit is less of a problem for the transmitter and the amp.

MFJ new units sometimes have problems with the receive splitter transformer so place the SDR on the transceiver position and note the backround noise using the SDR Program, then short the control line. This will compare the shared receive circuit to a direct connection. 3db is typical, any more than that and examine the magnet wires to be sure the insulation was removed properly before soldering.

My FT2000 does a better job switching antennas internally for the SDR, but if a rig does not have this, the MFJ-1708 is a cost effective solution. de Wb8yqj Don

N5IIA Rating: 5/5 Jan 22, 2019 04:42 Send this review to a friend
Works Great  Time owned: 0 to 3 months
The only complaint I have is how long it took to get the thing from MFJ. (Three months) No external vendors had them either, so it must be a popular item. I have it connected to my Icom 746PRO and an SDRPlay. Works like a champ. Coupled with HDSDR and Omni Rig, I have a very nice spectrum display, and can pounce on a signal just by clicking the mouse. Even better, I won't have to shell out a chunk of money to get a new wizz bang SDR transciever. So breathe life into that older radio you were contemplating selling or trading, and try this switch with your favorite SDR receiver and software. You'll be glad you did.
KE1MI Rating: 1/5 Jan 16, 2019 13:49 Send this review to a friend
Failure after 3 Months  Time owned: 0 to 3 months
I bought the MFJ 1708B SDR T/R switch back at Nearfest in the fall.
It did not survive whatever smoke test happened the other day. Kenwood transceiver, dipole, sdrplay. Catastrophic failure. Near as I can tell the relay that clamps the sdr during transmit failed, stuck open and smoked two resistors which it is designed to do. Good thing it did not smoke the sdrplay.
Hope to repair it sometime because it is 90 something dollars of junk.
I own an older 1708 non sdr that works good and have used that for a few years now with no problems, but it is simpler design. I would not trust the separation on it to the sdrplay, but works ok for boat anchors.
Just beware, probably poor components, and it is MFJ, which we all know suffers in the quality control department.
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