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Author Topic: MFJ 1788 for low profile questions  (Read 5937 times)
KA2ZEY
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Posts: 78




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« on: October 26, 2010, 01:37:30 PM »

Hello to all,
I have recently gotten back into the hobby and I'm shopping for radio/antenna combo. I live in an apartment in Brooklyn, NY on the first floor with a backyard. My landlord might allow me to have an antenna on the roof (3 stories) if it's very small. I'm looking at the 1788 to do 30 m thru 15 m during the day and 40 m at night (mostly CW and digital). Here are some questions I have for those who have used the 1788:

1) If I mount it on a painters poll up about 15 feet from the roof top, can I mount it horizontally?

2) I know it has to be tuned from just a small QSY. How long does it take to retune? For instance, let's say I'm at 7050 and decide to QSY to 7030, how long would that take to get the SWR back down?

3) is it possible to just tune around a band to see if there is action without having to tune? Will the receive be good enough to hear someone calling CQ and then I can tune or is it completely deaf until tuned?

4) If I have a 2m/440 vertical a few feet away and down from the Loop, will it effect the 1788's tunning?

5) What coax would be recommended to run up about 50 or 75' to the loop

Thanks,
KA9ZEY/2
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WB2WIK
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Posts: 20611




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« Reply #1 on: October 27, 2010, 02:06:37 PM »

Hello to all,
I have recently gotten back into the hobby and I'm shopping for radio/antenna combo. I live in an apartment in Brooklyn, NY on the first floor with a backyard. My landlord might allow me to have an antenna on the roof (3 stories) if it's very small. I'm looking at the 1788 to do 30 m thru 15 m during the day and 40 m at night (mostly CW and digital). Here are some questions I have for those who have used the 1788:

1) If I mount it on a painters poll up about 15 feet from the roof top, can I mount it horizontally?

Sure you can.  I've found they work better when installed vertically.  Maybe that's because they're bi-directional and using one vertically allows you to null noise in two directions.  But do what you can!

Quote
2) I know it has to be tuned from just a small QSY. How long does it take to retune? For instance, let's say I'm at 7050 and decide to QSY to 7030, how long would that take to get the SWR back down?

About one or two seconds.

Quote
3) is it possible to just tune around a band to see if there is action without having to tune? Will the receive be good enough to hear someone calling CQ and then I can tune or is it completely deaf until tuned?

Pretty deaf when not tuned.  If you have it tuned for 40m and QSY to 20m, good chance you won't hear anything!

Quote
4) If I have a 2m/440 vertical a few feet away and down from the Loop, will it effect the 1788's tunning?

Probably not.

Quote
5) What coax would be recommended to run up about 50 or 75' to the loop

High-quality RG8X would be fine.  (Not "Radio Shack" type, but Belden or a real brand-name cable manufacturer.)
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N3OX
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Posts: 8847


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« Reply #2 on: October 27, 2010, 05:58:01 PM »

Pretty deaf when not tuned.  If you have it tuned for 40m and QSY to 20m, good chance you won't hear anything!

But that's easy to fix... just tune until you hear something Grin

I can get my magnetic loop (homebrew, http://n3ox.net/files/magloopnew_lg.jpg) tuned to below 3:1 SWR just by peaking the received signal.  That's my coarse tuning indicator.

Wouldn't count the need to retune for reception when changing bands as much of a strike against it because it's so easy to peak up the receive.  I find that the most irritating thing is tuning for low enough SWR.  But if I were really using the thing day to day, I wouldn't mind much, because it works pretty great on 5-22MHz for a four foot octagon...

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KA2ZEY
Member

Posts: 78




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« Reply #3 on: October 27, 2010, 06:44:21 PM »

Hey thanks for the replies. I'm waiting for a yes or no vote from my landlord and his father whether or not I'll be able to place it on the roof. A few more questions here:

1) in the event that he says no to the roof, if I keep it in my back yard on a painters pole and mount it vertically, will I stil get decent results? Again, I am in an apartment surrounded by 2 and 3 story buildings

2) If I mount it on the roof and can get it up only 10' high from the chimney mount, is it ok to still mount horizontally? I am also considering a vertical mount because of the node. I'll have to invest in an inexpensive remote TV ant. rotor.

Thanks!

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WB2WIK
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Posts: 20611




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« Reply #4 on: October 27, 2010, 07:14:56 PM »

Hey thanks for the replies. I'm waiting for a yes or no vote from my landlord and his father whether or not I'll be able to place it on the roof. A few more questions here:

1) in the event that he says no to the roof, if I keep it in my back yard on a painters pole and mount it vertically, will I stil get decent results? Again, I am in an apartment surrounded by 2 and 3 story buildings

I've never actually operated under those condx...my experience with the MFJ loops is 100% in the field, at campgrounds and such without big buildings nearby.  However, I'd guess it will still work, probably not as well.

Quote
2) If I mount it on the roof and can get it up only 10' high from the chimney mount, is it ok to still mount horizontally? I am also considering a vertical mount because of the node. I'll have to invest in an inexpensive remote TV ant. rotor.

Yes, you'd want a rotator for it, since when mounted vertically it is directional.  I never had to do that, since I've used mine at parks, beaches, campgrounds, etc, so I could just turn it manually.  But a small rotator is a good idea. 

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W4FID
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Posts: 133




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« Reply #5 on: October 28, 2010, 04:39:19 AM »

I had mine on a TV mast -- unguyed -- at 20 feet. That's about 8 feet above the patio roof -- not even as "good" as you propose. However I had it vertically. Needs to be well in the clear to be horizontal. I had a TV rotor but found it wasn't really needed. The null is broadside to the plane of the loop but not too wide. So I left it "facing" in a direction I didn't care too much about and was able to work guys that direction anyhow. The painter's pole may not be quite strong enough ......... use a single length of TV mast.

It will need to be at least 5 feet away from anything that will detune it. 10 or 15 feet is good if you can. When it was in my attic for a while it was too close and 20M didn't tune very low ......... worked but SWR wouldn't go below about 2:1.

I could only move a few KHz on 30M and several on 20M --- maybe +/- 5 KHz on 30 and +/- 10 KHz on 20M. So plan to retune often. But it only takes about 5 or 10 seconds to get to the lowest point SWR. You tune by receiver noise till you're close if you change bands. Then use the cross needle meter to hit the low point. If you go from the CW portion to the phone portion there will be a very noticable drop in received strength and if you change bands it will be about deaf till you tune it.
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N3DF
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Posts: 252




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« Reply #6 on: October 28, 2010, 08:08:31 AM »

On 40 and 30 you will have significant negative gain (relative to a dipole) and likely marginal performance.  You will finder stronger signals, broader tuning and less noticeability with a fine wire 20 meter (33 ft) dipole and a tuner at the xmitr for higher bands.  You will also save perhaps $450.
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Neil N3DF
WB2WIK
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Posts: 20611




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« Reply #7 on: October 28, 2010, 09:43:37 AM »

I had mine on a TV mast -- unguyed -- at 20 feet. That's about 8 feet above the patio roof -- not even as "good" as you propose. However I had it vertically. Needs to be well in the clear to be horizontal. I had a TV rotor but found it wasn't really needed. The null is broadside to the plane of the loop but not too wide. So I left it "facing" in a direction I didn't care too much about and was able to work guys that direction anyhow. The painter's pole may not be quite strong enough ......... use a single length of TV mast.

It will need to be at least 5 feet away from anything that will detune it. 10 or 15 feet is good if you can. When it was in my attic for a while it was too close and 20M didn't tune very low ......... worked but SWR wouldn't go below about 2:1.

I could only move a few KHz on 30M and several on 20M --- maybe +/- 5 KHz on 30 and +/- 10 KHz on 20M. So plan to retune often. But it only takes about 5 or 10 seconds to get to the lowest point SWR. You tune by receiver noise till you're close if you change bands. Then use the cross needle meter to hit the low point. If you go from the CW portion to the phone portion there will be a very noticable drop in received strength and if you change bands it will be about deaf till you tune it.

I agree with all this from my own experiences with the same loop, which I've owned for several years now.  The loop resides in storage until we go camping or have some need for portable use.

I have compared the loop supported by a rope over a tree limb with various well-installed mobile whips including the larger screwdriver antennas, operating from RV parks and campgrounds and such, and the loop is better every single time on the bands it covers.  Being very narrow band, it tends to receive less noise as it strongly rejects noise and interference that's off frequency -- a real blessing when you're dealing with weaker signals.  Its transmitting performance is impressive, and with 100W output power I can work "almost" everything I can hear, and it hears quite a lot compared with mobile whips and such.

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KA2ZEY
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Posts: 78




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« Reply #8 on: October 28, 2010, 10:25:58 AM »

Hmmm, now I'm thinking of just building an OCF doublet to work 40 thru 10. I could run the doublet on one end tied to a mast on the roof and slope it down to about 8ft above ground tied to fence in the back of my yard. I would feed the antenna with 300 ohm ladder line running down along the side of my apartment building and into a balun to coax and then into the shack (with Ten Tec Jupiter tuner).

This means half of the antenna would not be in the clear sloped down in the backyard. I wonder if this would be a better setup than the loop.
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WB2WIK
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Posts: 20611




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« Reply #9 on: October 28, 2010, 03:07:13 PM »

Hmmm, now I'm thinking of just building an OCF doublet to work 40 thru 10. I could run the doublet on one end tied to a mast on the roof and slope it down to about 8ft above ground tied to fence in the back of my yard. I would feed the antenna with 300 ohm ladder line running down along the side of my apartment building and into a balun to coax and then into the shack (with Ten Tec Jupiter tuner).

This means half of the antenna would not be in the clear sloped down in the backyard. I wonder if this would be a better setup than the loop.

If it's 65-66 feet long and fed with the proper kind of transformer it should be.  I don't know why you'd want to use ladder line to feed an OCF doublet, as this is an unbalanced antenna.  Most OCF designs use a feedpoint transformer with coax.  A tuner is still required but the one in the Jupiter should handle it fine.
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N5YPJ
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Posts: 642




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« Reply #10 on: November 14, 2010, 09:03:17 AM »

I've had mine for a couple of weeks now. Let me say this it is a good stand alone antenna for certain restricted residences where one can't get a dipole a decent height above ground. Right now I'm antenna restricted, antenna height restricted - there is 8 to 10 ft above ground to be had. I put the loop on a 10 ft section of chain link fence rail anchored by an old lawn table base then go about making lots of contacts. I doubt it will be a properly installed dipole on any band and I'm not breaking pileups but even on 40 meters during WAE RTTY contest I made several European contacts. This antenna is expensive and perhaps it could have better features but in some situations there aren't many options. If you can get a wire or two or three on the 3 rd floor roof then you may do better with a dipole and save a bundle.
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K0KDS
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Posts: 1




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« Reply #11 on: December 21, 2010, 11:20:19 PM »

I've been looking at this antenna for several months now. I live in a first-floor, south-facing apartment in a two-story building. No access to the roof or attic. From what I've been reading, the 1788 would be my best bet for my situation. But how badly would it suffer with an indoor installation on the first floor? I heard it's just about useless on 40m. Has anyone had any issues with this on 40m?
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WB2WIK
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Posts: 20611




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« Reply #12 on: December 22, 2010, 10:25:03 AM »

I've been looking at this antenna for several months now. I live in a first-floor, south-facing apartment in a two-story building. No access to the roof or attic. From what I've been reading, the 1788 would be my best bet for my situation. But how badly would it suffer with an indoor installation on the first floor? I heard it's just about useless on 40m. Has anyone had any issues with this on 40m?

It's not useless on 40m, the antenna will certainly make contacts there.

First floor apartment with no attic or roof access-?  I'd install a mast in a tripod or a 5 gallon bucket filled with cement (or something) and use the mast to support the antenna outside, at ground level, when I'm using it, and then pull it back inside when I'm not using it.

Using the loop inside the apartment may yield poor results -- especially if the building construction includes bricks, concrete, stucco, aluminum or anything other than wood or vinyl.
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CT1DRB
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Posts: 6




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« Reply #13 on: December 22, 2010, 03:57:23 PM »

Hello all,

I got an MFJ-1786X some days ago and though to use it inside my balcony, or at least with help of a spiderbeam pole, outside my window. I am a bit disapointed with its performance up to now, but for sure I am not giving antenna best conditions. I tested it inside my balcony and got less swr on 30m, other bands were not possible to get fine swr. Last time I tested it hang on a treebranche, about 1m above ground, and got very disapointed performance on 30m. Other bands were impossible to tune. Next time will try it at higher than 1m from ground.

Best xmas for everyone and families included.

CT1DRB
David Quental
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