Slip Seat SWR Woes

[Old Sarge]

I have been trying to get a slip seat set up to function. I have welded two brackets to vise-grips for the antennas for each radio. I have a quick disconnect for the AM/FM antenna so it can be removed for SWR checks.

The CB is a Cobra 29 (actually I have two) The problem is the SWRs never come below 3:1. I have tried both radios. One worked at 2:1 in my wifes car with a trunk mount K40 and both work at 1:1 - 1.5:1 on a radio shack mag mount in my pick up.

I have tried a 5/8 silver stick (4') and a Diesel 3' with the slip seat and neither gets out of the red.

I have also tried a direct grount to the "bird perch" antenna mount by using jumper cables from the negative battery post to the "bird perch".

HELP!!

[kd5drx]

ok go get a 4ft francis and 18 ft of coax and problem solved.

[TomN]

i agree with KD5. I slip seat and i have cobra 29 with francis 5 ft antenna, with 18 ft coax.

[Old Sarge]

Thanks, What makes Francis work so much better than others? I have been told that Wilson is about the best there is for truck antenna.

I will probably have to order one on line as I don't know of any place around here that sells them.

Thanks again. I look forward to any other advise.

[kd5drx]

what makes francis so good is they are a real 1/4 wave antenna that is about as good as you can get they have no coils or traps or anything they have 1/4 wave length of copper wire wrapped around fiberglass and they radiate like the devil. THe trick is you have to use 18 ft of coax because they use the coax as the tuning stub to make it all match up. go with a 5 ft and you want be sorry.

[bbctransport]

I am going to make a few people just a little upset, but here goes.

1. A francis is actually a "double quarter-wave helical 'wound'" antenna. Meaning it is actually a coil from top to bottom that is ELECTRICALLY two quarter wave antennas phased end to end. That does not make it a half wave antenna either.

2. The reason they most always have a "good" SWR is , without referring you to several volumes of rf engineering material, is that within the entire length of the antenna power is being dissipated as heat and not radiated or "returned". Your SWR meter is basically showing you the amount of power being reflected versus the amount going to the antenna. Since the majority of your power is literally being wasted in the antenna very little remains to be reflected.

3. HOWEVER, they do help protect your radio in a situation like yours. Number 2 is also why they cover so many "channels".

4. Remember that to resonate an antenna on any frequency with a physically short antenna it must be "loaded" with either inductance or capacitance to make the short antenna "electrically" longer.

bbctransport
5 years with the SIGNAL!

[Old Sarge]

Thanks again for all the responses.

Here is my update.

I got a 4.5 ft Francis via e-bay (for less, including shipping than I just saw one at a truck stop in South Beloit). SWRs still in the red. I then purchased new coax, 18' of r58 (I had r8 and r59, both 18') and viola, 1.2:1 to 1.6:1 across the channels. No more antenna light and everyone seems to hear me just fine.

Thanks again, It is great to be able to talk without worrying about burning up my radio.

[BUZZSAW]

Old Sarge ,
Those old Francis antennas are hard to beat for the slip-seat ! I have used them since 1978 . They do pretty good provided you are getting a decent ground. I carry a 4' with me & I carry a 3 foot Wilson Slide to tune NO GROUND antenna ( this one comes with a special coax). Between these two - I can find something that WILL WORK !