AS1729vrc antenna woes

[Cav Trooper]

I finally got the RT246 up and running, tested on a stationary antenna and it works great. Installed the radio in my truck and waited for my radio guru to come over to tune it to the antenna on the truck. We could receive but no transmit to a PRC68B 3 ft. away. Checked the coax which was new, replace with another new one just in case. Finally removed the antenna from the tuning base the the center contact is broken off in the base of the antenna. So, now I need an AS1720 or a complete antenna assembly AS1729VRC. Anybody know where I can find one that doesn't cost as much as my radio? I saw that the AS3900 is a replacement but won't work with the MX6707.

 

[CARNAC]

The 3900 is NOT 12 series compliant.

You want either a replacement antenna (obviously) or go with the AS3916 which is the low profile/high survivability antenna. I think there was a post that this antenna should be mounted away from the radio.

Also, 3 ft away for radio test is pretty close. I recall our unit required a min 50 ft separation. Not sure if that's in the manual or anything.

 

[tennmogger]

I think that at 3 ft the PRC68B should be hearing the RT246 transmitting even with a bad antenna connection. Maybe the '246 is still not working? Your radio guru should bring a dummy load to test the transmitter.

Carnac, I'm curious about the not 'compliant' part. Do you mean the AS3900 was never issued with the RT-246? As for actual operation, the AS3900 is designed to present a reasonable load match across it's coverage range, but 'gain' is poor. The '3900 should be functional with the '246, just not with original performance.

Bob WB4ETT

 

[rickf]

Transmitting with no load (broken antenna) may have fried the finals in the transmitter. You need to hook up a dummy load and check to see if it is actually still transmitting.

Rick

 

[Cav Trooper]

I have a new antenna coming and my guru is bringing the dummy load to test with also. The radio was tested and working on his dummy load and then his antenna. My assumption was that the antenna I installed on the truck was good. I did not inspect it close enough to see that the center contact was missing. I did test the MX6707 for function and it works ok. We will see. I'm out of town until next Tuesday. So everything is on hold till then plus whenever the new antenna arrives. I understand that the AS3900 is not compatable with the MX6707.

 

[scottgs]

At 3 ft, I would think that you should be hearing the transmitter no matter what antenna problems there may be. Probably even if you have burned up finals too.

The AS 3900 should work fine. There may be some slight performance loss, but I bet you'd never notice it, for what you're likely to use it for. None of these antenna systems are particularly efficient.

Just take it one step at a time, and if you have two other working radios, then you can use the third radio as a verification. If you have a portable radio service analyzer or other test equipment, better yet. FWIW, my experience is that a large number of MX-6707's are bad. That's just they way they are.

Garret
W8BUG

 

[papakb]

There are a couple of things I'd like to address in this post.

Both the AS1729 and the AS3900 are antennas designed to work in the 30 - 75 Mhz range and should operate interchangably. The MX6707 in the 1729 antenna will give you slightly better antenna coupling because of the tuned elements inside and therefore a bit more power out. The 3900s typically have a bit more reflected power because they're designed to be broadband antennas and there's only a hefty ferrite in the base. VSWR will be a bit higher with this antenna so you'll lose a little output power there.


With ALL THAT POWER coming out of the 68B, maybe 2 watts, you may or may not have heard anything on the 246. If the 1729 antenna is properly attached to the base there's some good shielding in place and not much wiggle room for a signal to get through. To test the receiver all you need is a wire hanging off of the front panel BNC. Receivers typically don't care about impedance matching, they're just looking for a signal.

The 246 will receive just fine even of the final is totally blown out. This doesn't happen very often because the final is a Burle 7843 vacuum tube (or equivalent) and handles reflected power without much trouble. On the other hand, the finals in a 77 might be history! The finals are in the transmitter section and don't come into play in receive mode. There's also a T/R relay in there that switches when the transmitter is keyed up that protects the receivers front end.

Be aware that dummy loads only indicate transmitter output power to the antenna. They won't measure frequency or modulation and both of these need to be correct to transmit a signal to another radio.

I'm not clear on what you mean the center pin is broken off. Do you mean the center pin in the coax cable or the center pin of the input BNC to the matching unit? Your new cable should have corrected that issue if it's the center pin of the cable. I've seen the BNC on the 6707 fail when the center pin gets pushed into the unit. They also have a bad habit of corroding inside if water has gotten into them. They always told us never pressure wash or paint them because of this. There is a philips head screw on the bottom of the 6707 that you can remove to see if any water has gotten into the unit. The other thing that happens is vibration can cause the small coax inside the matching unit to break off. This is an easy repair but make sure to silicon seal the base if you open it up.

Hope some of this helpd you.

Kurt
KG6KMJ

 

[Flyingvan911]

My 1729 has much better range than the simple wire antenna. We here in KC have experimented with it.

 

[papakb]

My comment about wire antennas was that all the receiver needs to see is some type of antenna, not necessarily one that's tuned for the frequency.

Transmitters generating power definitely want to see an antenna that's frequency matched to it. This maximizes forward power (radiated power) and minimizes reflected power back into the radio (wasted power). Reflected power back into the radios can cause damage to the radio. A simple VSWR (visible standing wave ratio) meter is th istrument that we use to tune antennas. Ideally you would like a 1:0 ratio, with all the power radiating out the antenna but this is nearly impossible to achieve. Keeping the VSWR under 1:5 is good. We used an antenna analyzer recently to look at a 3900 and at 50Mhz it was almost 1:50.

Kurt

KG6KMJ

 

[scottgs]

VSWR = Voltage Standing Wave Ratio

VSWR is a good indicator of power coupling into the antenna, and with some antenna types, it can be loosely equated with antenna resonance. With these military antenna "systems", that's not exactly the case. These antennas are non-resonant antennas (except at one narrow band), combined with an antenna matching unit. The VSWR will not really tell you for certain if the antenna system is working properly, but it is, in most cases a good first test. In other words, if you've got a bad VSWR, then you definitely have a problem, but you could have a good VSWR and still have an antenna system problem, but it's less likely.

I wonder if CAV Trooper can post a photo of his broken center pin? I'm having trouble visualizing this. I also am curious what type of meter he's using. Even a CB SWR meter should work with this, as long as you have proper adapters.

Garret
W8BUG

 

[rickf]

My comment about wire antennas was that all the receiver needs to see is some type of antenna, not necessarily one that's tuned for the frequency.

Transmitters generating power definitely want to see an antenna that's frequency matched to it. This maximizes forward power (radiated power) and minimizes reflected power back into the radio (wasted power). Reflected power back into the radios can cause damage to the radio. A simple VSWR (visible standing wave ratio) meter is th istrument that we use to tune antennas. Ideally you would like a 1:0 ratio, with all the power radiating out the antenna but this is nearly impossible to achieve. Keeping the VSWR under 1:5 is good. We used an antenna analyzer recently to look at a 3900 and at 50Mhz it was almost 1:50.

Kurt

KG6KMJ

I think you need to read up on swr a bit, The ratio to look for is 1:1, 1: 5 is bad and 1:0 is impossible.
VSWR conversion calculator

 

[tennmogger]

I think you need to read up on swr a bit, The ratio to look for is 1:1, 1: 5 is bad and 1:0 is impossible.
VSWR conversion calculator

Kurt knows his stuff.. it's just a punctuation issue. IMHO, his 1:5 was meant to be 1.5 to 1 ratio, 1:1 is 1.1 to 1, etc.

Bob WB4ETT

 

[Cav Trooper]

My guru was using a meter that was home built. The 68b would almost peg the meter on a low setting, the 246 would not even register. I just received my new antenna, the center contact is there which was missing on the old antenna. There are 2 contacts in the base of the 1729, the horseshoe or "U" shaped copper contact and the center which looks like a hollow piece of metal tubing about 3/16" dia. which is supposed to make contact with the center conductor on the spring mounted antenna base. The copper "U" shaped contact is supposed to mate to the ring around the center contact of the spring base. I got this antenna off ebay, NOS shipped from Israel, only about a 10 days. Now for some time to retest the system.