By two resonances are you referring to the peak at around 156MHz? If so, th= at is a peak from a separate hand-held unit on channel 1 with the PTT butto= n pressed. The coupler is a -30dB one I don't actually remember what the tracking generator was set to, but it wa= s less than 0dBm. I'll look at the setting on that tomorrow. I have a diffe= rent antenna I'm trying at home & I know that I can set the TG on my own an= alyser to 0dBm. First results indicate the dip is centered on channel 16. Do you know of a method for matching based on co-ax length? There are minim= um lengths I can use due to the locations of antenna & radio. Changing leng= ths is a pain as I have to re-do the radio or antenna connection each time.= Anyone know how sensitive the cable layout is? It seems to make a differen= ce whether it is placed beside metallic objects. Such routing is unavoidabl= e. There is also seawater involved which can sometimes partly cover the cab= le. (This is in a rigid-hull inflatable & the cables run along the deck fro= m the console to the antenna which is mounted on top of a steel frame on th= e stern.) Re tagging this with EE as per your suggestion & leaving the original mails= below so that anyone on EE only can follow. Thanks, Joe ________________________________________ From: piclist-bounces@mit.edu [piclist-bounces@mit.edu] On Behalf Of Sean B= reheny [shb7@cornell.edu] Sent: 03 June 2014 17:41 To: Microcontroller discussion list - Public. Subject: Re: [OT] Matching an antenna to a VHF TX/RX Hi Joe, You might try posting this under EE as well. Can you tell me how much power the tracking gen is outputting and what the coupling of the directional coupler is? It is difficult to say how good these antennas are based on the plots - if, for example, your tracking generator were producing +10dBm and the coupler was a -20dB one, then these plots would indicate that both antennas are pretty well matched over the entire 140 to 170MHz range (return loss would be about 20dB across the whole band). If the generator is producing 0dBm and your coupler is a -30dB one, then these antennas are rather poor except in a very narrow band, as you suggest. What type of antennas are these? I notice that there appear to be two resonances - one which minimizes the return loss and the other which maximizes it. Seems like maybe the main resonant impedance of the antenna might not be 50 ohms, so that the matching is coax length dependent. Sean On Tue, Jun 3, 2014 at 11:44 AM, Joe McCauley wrote: > I'm fitting new antennas to 2 marine VHF radios. I did a check using a > spectrum analyser & tracking generator together with a directional couple= r > to measure return loss from the antenna. On the analyser I see a valley a= t > ~153.2 MHz indicating that the antenna/RG58 cable is tuned to this > frequency. The radios actually transmit at a higher frequency > (approximately 156 and 162 MHz). > I would have thought that this range is where I should see the 'dip' in > the spectrum. The other thing I notice is that one of the antennas shows = a > much deeper dip & has the dip at a slightly higher frequency of ~153.8MHz= .. > Plots are attached showing the dip with each antenna, together with a > transmission peak on channel 1 from a hand-held TX. > > The antenna instructions (such as they were) indicated that it was a plug > & play type of situation. I'm wondering would it help to tune the antenna > by trimming the length, but am wary of doing so. Anyone have experience > with stuff like this? We find that reception is poor on one of the boats, > but whether this is down to poor reception in that boat or poor > transmission in the other is hard to say. > > Thanks for any pointers, > > Joe > > > -- > 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 .