Joe, what kind of antenna is this? In an ideal situation, the coax length or routing should not make a difference in the power radiated by the antenna (other than the additional loss if the coax is lengthened). If there is an impedance mismatch, then the length of the coax will change the impedance seen by the transmitter. If the antenna is an inherently balanced type but is being fed directly by coax (with no balun), then the radiation pattern will be distorted and will depend on the coax routing. If the antenna is an unbalanced type (like a vertical intended to work with a ground plane), then it can have a distorted pattern AND a different impedance if the ground plane/radials are insufficient. The coax shield may then be acting as part of the effective ground plane and the routing will affect its operation. On Wed, Jun 4, 2014 at 11:24 AM, Joe McCauley wrote: > I could if I had one :). Doing this also means having to break into the > coax. I probably want to try with a new coax anyhow. I did some tests wit= h > an old antenna last night. In general I noted that the dip in the reflect= ed > power was pretty much where I expected it to be, but there was a change i= n > both dip frequency & amplitude depending on where the coax was routed. > > I'll also see if I can find a meter. Boats are being launched soon, so I > probably won't get to do this until we have them back on dry land. > > Thanks, > > Joe > > > -----Original Message----- > From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf > Of Sean Breheny > Sent: 04 June 2014 15:52 > To: Microcontroller discussion list - Public. > Subject: Re: [OT] Matching an antenna to a VHF TX/RX > > Joe - you could simply put a directional power meter in-line with the > antenna, right at the antenna feedpoint to eliminate the effect of the > coax. The forward power minus the reflected power will be the actual powe= r > delivered to the antenna. Assuming that the antenna is not faulty or > installed near objects which absorb power, this will be close to the actu= al > radiated power. Beyond that, the only remaining variable is antenna patte= rn. > > > On Wed, Jun 4, 2014 at 6:44 AM, Joe McCauley wrote: > > > Does a spectrum analyser with tracking generator & directional coupler > > not do the same thing? (Measuring reflected energy from the antenna.) > > I have the TG output connected to the through port of the coupler & > > the antenna connected on the other side. The coupled port of the > > coupler is connected to the RF input of the analyser. > > > > Thanks, > > > > joe > > > > -----Original Message----- > > From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On > > Behalf Of Vasile Surducan > > Sent: 04 June 2014 09:17 > > To: Microcontroller discussion list - Public. > > Subject: Re: [OT] Matching an antenna to a VHF TX/RX > > > > If you have a network analyzer will be much easier. Just connect the > > antenna on one port (with the whole cable you are using between the > > antenna and the input port) and see the S11. Then tune the antenna > > lenght (or adjust the trimmer capacitor if has one) until the max > > absorption appears to be on your required frequency. > > > > I think between 153 and 156 MHz you should do nothing unless you have > > small emitting power. > > > > > > On Wed, Jun 4, 2014 at 6:02 AM, Justin Richards > > > > > > wrote: > > > > > On 4 June 2014 03:38, Joe McCauley wrote: > > > > > > > That is the next step I think, how far apart do you put the > > > > antennas? I'm guessing they can't be too far apart. > > > > > > > > They are close, approx 20 m. If you had a portable B that you > > > > could > > > position the same distance from each and save trace to disk so you > > > can move analyser to the other boat. > > > > > > I recall that the farming community I once belonged to were adamant > > > that a two way radio (UHF) would fail to work if the antenna was > > > mounted on the roof of a vehicle and the two way radio was mounted > > > directly under the antenna inside the cab. > > > > > > justin > > > -- > > > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > > > View/change your membership options at > > > http://mailman.mit.edu/mailman/listinfo/piclist > > > > > -- > > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > > View/change your membership options at > > http://mailman.mit.edu/mailman/listinfo/piclist > > > > -- > > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > > View/change your membership options at > > http://mailman.mit.edu/mailman/listinfo/piclist > > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .