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Author Topic: 102" Whip and 75 Meter performance  (Read 1970 times)
AJ3U
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« on: December 31, 2003, 02:41:19 PM »

Hello all,

My current mobile setup is a 706MKIIG, AH-4 Tuner and 102" Whip. I have had good results on the upper bands, but I'm having trouble with 75 meters. The AH-4 tunes up fine on 75 and the receive is great. Many stations I hear are 59, however, when I transmit, if I'm heard at all, I get "you're down in the noise, OM! - Can't copy!" I know my mic gain and radio is set up correctly because of my performance on other bands so I can only assume it is lack of efficiency of the antenna system on 75m.

I understand that I'm pushing the limits of my setup by trying to get on 75 meters, but I'm looking for some tips to improve performance. My main interest is to check into the 3905CCN net in the evenings and I will be at my home QTH at the time of the net so I can modify my mobile setup and operate "portable" instead.

I was thinking of extending the length of the 102" whip by using a random wire attached to the top of the whip and going up to the peak of my two story house. This type of setup was suggested in the AH-4 manual. Before I go climbing up on the roof to try it, does anyone have experience with this type of configuration? It seems easy enough, and I could just unhook the end connected to the whip and drive away. I'm in a antenna restricted community so I'm unable to put up anything elaborate.

A very basic drawing of what I have in mind is here:

http://www.aj3u.com/images/randomwire-idea.jpg

Thanks for the help and 73!!

A.J. Farmer
AJ3U
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N8EMR
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« Reply #1 on: December 31, 2003, 03:35:14 PM »

you not going to get a stock 102" whip on 75m, at least and not have a usable signal. Its just too short with the AH-4, You can do it with some of the SGC tuners but again its still pretty short.

If you add some extenders onto the whip and legnthen the overall legnth 6+ feet it will work. I have seen whips with wire entenders attached and run to the front  of a vehicle. You get both a longer antenna and a guy wire to keep it vertial at speed.

I have a 16ft whip and the ah-4 will tune it on 75 and give me a usable signal. It however is a bit large for mobile use even whip pulled down. The easiest way is a quick connect and swap in 75m hamstick.

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WB2WIK
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« Reply #2 on: December 31, 2003, 05:51:08 PM »

I don't know why a 75m Hamstick would be any more efficient than a 102" whip with a tuner at its base.  The 75m Hamstick is still a 7' antenna made of a bunch of wire coiled up...but only the 7' length radiates.

I've used the "clip a wire onto the end of the whip" approach several times, and this does work well.  Never had an AH-4, but with my old SGC-230 and 102" whip (with one inch of HV wire interconnecting the tuner's output terminal with the ball mount for the whip), while the tuner will load the whip, its performance stinks.  Clipping 10' or 20' of insulated wire to the tip of the whip using an "alligator" clip and tossing the wire almost anywhere helps it a great deal.  The more wire you clip on, the better it works, and the more bandwidth you achieve before the tuner starts clicking again...

WB2WIK/6
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AJ3U
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« Reply #3 on: December 31, 2003, 07:01:08 PM »

Thanks guys for the input.  After reading some more posts in this forum, I went for it.  I cut 60' of 14 gauge insulated wire (1/4 wavelength of 75 meters) and connected one end to the peak of my house (with an insulator of course).  On the other end, instead of connecting it to the tip of the 102" whip, I connected it to a quick disconnect since I already had a quick disconnect setup for my whip.  I got that tip from another post and it makes perfect sense.  All I have to do is disconnect my whip and connect the long wire.  It's quick and easy.

I tuned up and made two contacts - both gave me a 59+.  With one guy, I switched between the whip and the long wire to compare them - he said I was booming in with the long wire, but he could barely hear me above the noise with the whip.

Looks like I can check into that net now!  

73!

AJ3U
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AA4PB
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« Reply #4 on: December 31, 2003, 11:16:59 PM »

I don't believe the AH-4 is designed to match a 102-inch whip on 75M. Even if it finds a match it is likely to be very inefficient. The SGC SG-237 is designed to load a 102-inch whip on 75M. If its properly installed clear of nearby metal, has a very short feed wire, and a proper mount insulator then the 102-inch whip will work as well on 75M as a Hamstick. I've done A/B tests swapping the Hamstick (tuner bypassed) for the 102-inch whip with tuner and found no difference in signal strength.

One solution for you with the AH-4 might be to put on a 75M Hamstick when you want to get on 75M. Replace it with the 102-inch whip for other bands. On 75M the Hamstick will present the tuner with a 25 ohm or so load and be much easier for the AH-4 to match efficiently.
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AC2RC
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« Reply #5 on: January 02, 2004, 11:17:07 PM »

After a friend demonstrated the huge difference between 102" whip and 75m hamstick on 80m , with AH-4 and 706, I immediately bought a 75m hamstick. I now run a 102" whip 40-6m and 75m hamstick for 75-80m.
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WA4MJF
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« Reply #6 on: January 04, 2004, 09:27:32 PM »

It seems like you would need a heavier
gauge wire to drag your house down the
road, but hey it works!

:-)

73 de Ronnie

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KI5DR
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« Reply #7 on: January 14, 2004, 11:08:26 PM »

I know of a ham that had a 102" whip attached to his rear bumper, and another one attached to the front of his pickup.

He fed the rear 102" whip, but ran a piece of wire from the tip of the rear whip to the tip of the front whip, with the front whip not terminated or connected to anything.

Effectively, he had a 26' random wire while mobile.

Just a thought...

73!
Scott - KI5DR
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W5DXP
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« Reply #8 on: January 22, 2004, 01:10:17 AM »

I tested a 138 inch whip with an SCG-230 on 75m at a mobile shootout in CA. It was 12 dB down from the screwdrivers and bugcatchers. That's 1/16 as efficient. A 102 inch whip would be even worse.
--
73, Cecil, W5DXP
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
N3JWN
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Posts: 103




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« Reply #9 on: January 31, 2004, 07:42:35 PM »

OK well the installation is 706MKII2G/w AH-4 Tuner &102" Whip (NO GOOD FOR 75-160) in a 98 Jeep charokee the radio is under the passenger seat and the tuner is under the back seat so the tuner is really just 3 feet away from the radio and the seat belt bolt I used for the ground is less than one foot away from the tuner!! Tied to the gnd common point on the tuner with 2" braid and as short as possible all control/ remote cables and coaxes are run with Troind Core and wrapped as many turn as i could get and as close to every plug input or output including control and remote Cables and the even the power Cables ( helps keep RF out of truck and wiring ). Antenna is fed with 5-6 ft of rg8X coax and a choke wound 6" from the connector to a 102" whip. But have you tested the radio and tuner on a dummy load or on the bench with an outside antenna are you sure the rig and tuner works.......... also I recommend #6-8 wire directly to the Battery Please fuse both pos and neg leads as close to the battery as possible for safety and to avoid a fire!!!! Too small a wire will give you a DC loss, at the Radio and since the tuner gets its power form the separate 706 tuner output regulator chip a 2 volt drop (from too small a wire from the battery) will probably cause the tuner and the 706 not to work properly!! I don't know what else to tell you. It takes care and patients for a good installation I have absolutely no noise from my vehicle but occasionally I get ignition noise from other cars around me and Outside motor noise on AM!!Will not tune very well on 75-80 after all what do you want with 102" whip I suggest the Bug Catcher or a screwdriver antenna!!! I can work 40-6 meters @100watts with no problem at all!!Be safe and have fun Dick / n3jwn
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N3JWN
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« Reply #10 on: January 31, 2004, 07:53:38 PM »

I think we're lucly we get the great results from That AH-4 Tuner fron 6 thru 20 Meters Fantastic . But in reality we;re expecting too much from Just 102" of wire the Q of that antenna below 20 meters is terrible forgot the figure. So if you want to work 40-80 as well you must get a higher Q antenna go to the Carolina Bug catcher or the High Seriah antenna because you need it there!! good luck have fun and be safe!! Dick/ n3jwn............................73's
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N3JWN
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« Reply #11 on: March 17, 2004, 11:11:55 AM »

I totally forgot to mention with current setup and no changes I run a 75 meter Hamstick and it works 75-80 meters and the ah-4 tunes even to 160 meters and I can work both banda at vert little swr with a full 100 watts But rec. sensitivity cut severely on the rsst of the band but for 80 great  anyother Band I have to got ham stick for that band or the 102' whip...............Dick / n3jwn
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KA9KQH
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« Reply #12 on: February 23, 2005, 08:19:43 PM »

<<------begin quote ----->>>>>>
also I recommend #6-8 wire directly to the Battery Please fuse both pos and neg leads as close to the battery as possible for safety and to avoid a fire!!!!
<<------end of quote ---->>>>>>

While I concur with the gauge of wire you recommend,it was recently pointed out to me that fusing the negative lead really serves no purpose other than to guarantee that if the negative lead shorts and blows its fuse that the radio will pick up it's ground though the antenna system or the chassis ground and you won't realize you had a problem till the radio fries on you. So the only fuse that is going to really count is the fuse in the positive lead.

just my two cents
73 de KA9KQH
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